7-digit hashes are bogus, so I run with core.abbrev = 12.

Non-default settings for core.abbrev caused the git blame test to fail,
because it was hard-coded to look for exactly 7 digits. This change
allows an --abbrev option to be passed to the git-blame wrapper, and
ensures that the same value is used for the git-blame operation and the
computed expectation.

TEST=tests/git_common_test.py GitReadOnlyFunctionsTest.testBlame

Change-Id: I83cbf4dd7267ea36607119bef52f303d59c3f840
Reviewed-on: https://chromium-review.googlesource.com/451124Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Mark Mentovai <mark@chromium.org>