Add detection for locked git repos.
Add detection for locked git repos.

R=mithro@mithis.com, mmoss@chromium.org, vadimsh@chromium.org
BUG=405426

Review URL: https://codereview.chromium.org/1498203002

git-svn-id: fd409f4bdeea2bb50a5d34bb4d4bfc2046a5a3dd@297850 0039d316-1c4b-4281-b951-d872f2087c98

duanwujie/depot_tools,primiano/depot_tools,aleonliao/depot_tools,duanwujie/depot_tools,hsharsha/depot_tools,CoherentLabs/depot_tools,aleonliao/depot_tools,duongbaoduy/gtools,duongbaoduy/gtools,CoherentLabs/depot_tools,primiano/depot_tools,hsharsha/depot_tools,primiano/depot_tools,hsharsha/depot_tools,duanwujie/depot_tools,aleonliao/depot_tools,duongbaoduy/gtools