Fix hook disable logic in gclient_scm (non-atomic os.rename on Windows)

Fix hook disable logic in gclient_scm (non-atomic os.rename on Windows)

crrev.com/348703002 has introduced some code into gclient_scm.py which
disables the .git/hooks when gclient is running in managed mode.
The disabling logic renames the individual hook files to hook.disabled
using os.rename. Conversely to what happen on Posix OSs, on Windows
os.rename does not have atomic rename semantics [1] and it fails if the
destination file already exists.
This change improves the hook disable logic.

[1] See https://bugs.python.org/issue8828 and os.replace in Python 3

BUG=474218
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=294715

[Primiano Tucci (use gerrit), 2015-04-07 09:09:52]

primiano@chromium.org changed reviewers:
+ szager@chromium.org, thakis@chromium.org

[Nico, 2015-04-07 16:56:56]

lgtm

[szager1, 2015-04-07 17:37:50]

lgtm

[Primiano Tucci (use gerrit), 2015-04-08 07:48:36]

The CQ bit was checked by primiano@chromium.org

[commit-bot: I haz the power, 2015-04-08 07:48:52]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1063973002/1

[commit-bot: I haz the power, 2015-04-08 07:51:59]

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

[commit-bot: I haz the power, 2015-04-08 07:52:00]

Presubmit check for 1063973002-1 failed and returned exit status 1.

Running presubmit commit checks ...
Checking out rietveld...
Running save-description-on-failure.sh
Running push-basic.sh
Running upstream.sh
Running submit-from-new-dir.sh
Running abandon.sh
Running submodule-merge-test.sh
Running upload-local-tracking-branch.sh
Running hooks.sh
Running post-dcommit-hook-test.sh
Running upload-stale.sh
Running patch.sh
Running basic.sh

** Presubmit ERRORS **
Missing LGTM from an OWNER for these files:
    depot_tools/gclient_scm.py

Presubmit checks took 182.2s to calculate.

[jochen (gone - plz use gerrit), 2015-04-08 09:02:52]

jochen@chromium.org changed reviewers:
+ jochen@chromium.org

[jochen (gone - plz use gerrit), 2015-04-08 09:02:52]

lgtm

[Primiano Tucci (use gerrit), 2015-04-08 09:11:22]

The CQ bit was checked by primiano@chromium.org

[commit-bot: I haz the power, 2015-04-08 09:11:41]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1063973002/1

[commit-bot: I haz the power, 2015-04-08 09:14:50]

Committed patchset #1 (id:1) as
http://src.chromium.org/viewvc/chrome?view=rev&revision=294715