This CL makes gclient understand correctly whether a git project is
being moved from DEPS to an upper project and vice-versa.
The driving use case for this is the upcoming Blink merge, where
third_party/Webkit will be removed from DEPS (and .gitignore) and will 
become part of the main project.

At present state, gclient leaves the .git folder around when a project
is removed from DEPS, and that causes many problems. 

Furthermore this CL solves the performance problem of bisecting across
the merge point. The subproject's (Blink) .git/ folder is moved to a
backup location (in the main checkout root) and is restored when moving
backwards, avoiding a re-fetch when bisecting across the merge point. 

BUG=431469

Review URL: https://codereview.chromium.org/743083002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@293329 0039d316-1c4b-4281-b951-d872f2087c98