Reland of Simplify the initialization of Git objects in Host.

Reland of Simplify the initialization of Git objects in Host. (patchset #1 id:1 of https://codereview.chromium.org/2671583002/ )

Reason for revert:
Reverting speculative revert; the flaky failure on Mac was unrelated to this CL.

Original issue's description:
> Revert of Simplify the initialization of Git objects in Host. (patchset #3 id:40001 of https://codereview.chromium.org/2663623003/ )
> 
> Reason for revert:
> Speculative revert. This might be the cause of build flakiness, see crbug.com/687707
> 
> Original issue's description:
> > Simplify the initialization of Git objects in Host.
> > 
> > This CL makes calling Host.initialize_scm() unnecessary by making
> > it so that the Git object is initialized when required. It also
> > merges scm() and scm_for_path(), and moves initialization logic
> > into the Git class.
> > 
> > BUG=676012
> > 
> > Review-Url: https://codereview.chromium.org/2663623003
> > Cr-Commit-Position: refs/heads/master@{#447582}
> > Committed: https://chromium.googlesource.com/chromium/src/+/7091f37e5b763d57e4752a4e69cf46719b8ea1c4
> 
> TBR=jeffcarp@chromium.org,qyearsley@chromium.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=676012
> 
> Review-Url: https://codereview.chromium.org/2671583002
> Cr-Commit-Position: refs/heads/master@{#447650}
> Committed: https://chromium.googlesource.com/chromium/src/+/6cd1e0590f82ef11669abfbf00d846d759235460

TBR=jeffcarp@chromium.org,lushnikov@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=676012
Review-Url: https://codereview.chromium.org/2676673006
Cr-Commit-Position: refs/heads/master@{#448196}
Committed: https://chromium.googlesource.com/chromium/src/+/d445ec6cb5feb192e8ad5cb7d23bdac8d5a4a188

[qyearsley, 2017-02-04 02:02:10]

The CQ bit was checked by qyearsley@chromium.org

[qyearsley, 2017-02-04 02:02:11]

Created Reland of Simplify the initialization of Git objects in Host.

[commit-bot: I haz the power, 2017-02-04 02:03:07]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-02-04 05:33:41]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2017-02-04 05:33:42]

Try jobs failed on following builders:
  linux_chromium_rel_ng on master.tryserver.chromium.linux (JOB_TIMED_OUT, no
build URL)

[jeffcarp, 2017-02-04 23:45:39]

lgtm

[qyearsley, 2017-02-05 17:09:16]

The CQ bit was checked by qyearsley@chromium.org

[commit-bot: I haz the power, 2017-02-05 17:09:27]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-02-06 00:37:06]

CQ is committing da patch.

Bot data: {"patchset_id": 1, "attempt_start_ts": 1486314556672170, "parent_rev":
"11f780768683d40c8a159698c321707a299dbf1c", "commit_rev":
"d445ec6cb5feb192e8ad5cb7d23bdac8d5a4a188"}

[commit-bot: I haz the power, 2017-02-06 00:37:41]

Description was changed from

==========
Reland of Simplify the initialization of Git objects in Host. (patchset #1 id:1
of https://codereview.chromium.org/2671583002/ )

Reason for revert:
Reverting speculative revert; the flaky failure on Mac was unrelated to this CL.

Original issue's description:
> Revert of Simplify the initialization of Git objects in Host. (patchset #3
id:40001 of https://codereview.chromium.org/2663623003/ )
> 
> Reason for revert:
> Speculative revert. This might be the cause of build flakiness, see
crbug.com/687707
> 
> Original issue's description:
> > Simplify the initialization of Git objects in Host.
> > 
> > This CL makes calling Host.initialize_scm() unnecessary by making
> > it so that the Git object is initialized when required. It also
> > merges scm() and scm_for_path(), and moves initialization logic
> > into the Git class.
> > 
> > BUG=676012
> > 
> > Review-Url: https://codereview.chromium.org/2663623003
> > Cr-Commit-Position: refs/heads/master@{#447582}
> > Committed:
https://chromium.googlesource.com/chromium/src/+/7091f37e5b763d57e4752a4e69cf...
> 
> TBR=jeffcarp@chromium.org,qyearsley@chromium.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=676012
> 
> Review-Url: https://codereview.chromium.org/2671583002
> Cr-Commit-Position: refs/heads/master@{#447650}
> Committed:
https://chromium.googlesource.com/chromium/src/+/6cd1e0590f82ef11669abfbf00d8...

TBR=jeffcarp@chromium.org,lushnikov@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=676012
==========

to

==========
Reland of Simplify the initialization of Git objects in Host. (patchset #1 id:1
of https://codereview.chromium.org/2671583002/ )

Reason for revert:
Reverting speculative revert; the flaky failure on Mac was unrelated to this CL.

Original issue's description:
> Revert of Simplify the initialization of Git objects in Host. (patchset #3
id:40001 of https://codereview.chromium.org/2663623003/ )
> 
> Reason for revert:
> Speculative revert. This might be the cause of build flakiness, see
crbug.com/687707
> 
> Original issue's description:
> > Simplify the initialization of Git objects in Host.
> > 
> > This CL makes calling Host.initialize_scm() unnecessary by making
> > it so that the Git object is initialized when required. It also
> > merges scm() and scm_for_path(), and moves initialization logic
> > into the Git class.
> > 
> > BUG=676012
> > 
> > Review-Url: https://codereview.chromium.org/2663623003
> > Cr-Commit-Position: refs/heads/master@{#447582}
> > Committed:
https://chromium.googlesource.com/chromium/src/+/7091f37e5b763d57e4752a4e69cf...
> 
> TBR=jeffcarp@chromium.org,qyearsley@chromium.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=676012
> 
> Review-Url: https://codereview.chromium.org/2671583002
> Cr-Commit-Position: refs/heads/master@{#447650}
> Committed:
https://chromium.googlesource.com/chromium/src/+/6cd1e0590f82ef11669abfbf00d8...

TBR=jeffcarp@chromium.org,lushnikov@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=676012

Review-Url: https://codereview.chromium.org/2676673006
Cr-Commit-Position: refs/heads/master@{#448196}
Committed:
https://chromium.googlesource.com/chromium/src/+/d445ec6cb5feb192e8ad5cb7d23b...
==========

[commit-bot: I haz the power, 2017-02-06 00:37:42]

Committed patchset #1 (id:1) as
https://chromium.googlesource.com/chromium/src/+/d445ec6cb5feb192e8ad5cb7d23b...