Get lastchange.py to work correctly on Git repositories.

Get lastchange.py to work correctly on Git repositories.

This changes lastchange.py in two ways:
1) If the commit it finds is a Git hash, it outputs the whole hash, not just
   the first 7 characters.
2) It only looks at HEAD to see if there is a git-svn id. Previously, it used
   --grep=git-svn-id, which would find the most recent commit containing a
   git-svn id. This would be broken after the switch to git, as it would always
   find the last commit before the switch. Now, it only inspects the most recent
   commit, and falls through to pure-Git if that fails.

R=dilmah@chromium.org, stip@chromium.org
BUG=399113

Review URL: https://codereview.chromium.org/488733002

Cr-Commit-Position: refs/heads/master@{#291165}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@291165 0039d316-1c4b-4281-b951-d872f2087c98