Make automated branch creation gnumbd-save.

Make automated branch creation gnumbd-save.

This makes now the same simplification as the chromium
release scripts do. For creating branch B from a gnumbd'ed
(aka real) commit X do:
1. Branch Y off the real X
2. Set refs/pending/heads/B to Y
3. Set refs/pending-tags/B to X
4. Set refs/heads/B to X

The old algorithm tried to branch off the pending
correspondent of X. That commit was determined by comparing
tree objects of the real X and commits on pending.
Unfortunately, multiple commits on one branch can refer to
the same tree object, e.g., for commits P, Q, R with R being
the revert of Q, P and R refer to the same tree object.

TBR=tandrii@chromium.org
NOTRY=true
TEST=./script_test.py
TEST=tools/release/create_release.py -a me -r you --dry-run

Committed: https://crrev.com/934dd88d3b2cee6d3952676fc89844c41b77adf2
Cr-Commit-Position: refs/heads/master@{#27042}

[Michael Achenbach, 2015-03-06 09:06:32]

machenbach@chromium.org changed reviewers:
+ hablich@chromium.org, tandrii@chromium.org

[Michael Achenbach, 2015-03-06 09:08:09]

PTAL

[Michael Achenbach, 2015-03-06 13:01:53]

The CQ bit was checked by machenbach@chromium.org

[commit-bot: I haz the power, 2015-03-06 13:10:16]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/979243004/1

[commit-bot: I haz the power, 2015-03-06 13:10:27]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-06 13:10:44]

Patchset 1 (id:??) landed as
https://crrev.com/934dd88d3b2cee6d3952676fc89844c41b77adf2
Cr-Commit-Position: refs/heads/master@{#27042}

[tandrii(chromium), 2015-03-09 16:49:54]

LGTM

Yay, simlified :)

[agable, 2015-03-09 17:59:05]

On 2015/03/09 at 16:49:54, tandrii wrote:
> LGTM
> 
> Yay, simlified :)

Nice