Many people* have complained on chromium-dev about the long times
required to perform a full fetch over a DSL. This seems to be mostly
due to the huge size of chromium's history (~9 GB). On the other side,
not everybody is interested in downloading the full git history of
the projects. The size of git packs required to fetch a working HEAD
is one order of magnitude smaller (1.5 GB).
This change makes it possible to perform a shallow fetch (in a way
which is consistent with DEPS, leveraging git templates on clone),
reducing fetch times by 80% for those not interested in the history.

* See: 
[chromium-dev] "fetch chromium" keeps hanging/getting stuck on Windows 7
[chromium-dev] Initial checkout with git taking long
[chromium-dev] Trying to get latest source code fails when fetching
[chromium-dev] Gclient sync takes too long

BUG=228996

Review URL: https://codereview.chromium.org/437903002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@287606 0039d316-1c4b-4281-b951-d872f2087c98