Add a git mode parameter for bot_update
Add a git mode parameter for bot_update

This is because --revision HEAD is ambiguous as to whether or not we want to
output a git or SVN got_revision.

BUG=361346

Review URL: https://codereview.chromium.org/258433002

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@265761 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build