Until recently, no proper Git clone of depot_tools existed.
All Git clones were set up locally via git svn clone.  The
way repos of that type are updated is via 'git svn rebase'.

Now, a supported Git clone of depot_tools exists.  The
update process for repos of this type is via 'git fetch'
and 'git rebase'.

Modify the update_depot_tools* scripts so they can update
depot_tools for git cloned or git svn cloned methods.  This
also means that git svn is no longer required, so move that
test after we check for git cloned repos.

R=maruel@chromium.org

TEST=Linux + git clone: on Linux, git clone
https://git.chromium.org/chromium/tools/depot_tools.git.  Apply
and commit this patch locally.  Note commit hash of HEAD.  Run
'git reset --hard "HEAD^^^"'.  Run
'git cherry-pick HASH_OF_UPDATE_CHANGE'.  Run './update_depot_tools'.
Verify depot_tools is updated to latest available revision + this
change.

TEST=Linux + git svn clone: on Linux, git svn clone
svn://svn.chromium.org/chrome/trunk/tools/depot_tools.
Apply and commit this patch locally.  Note commit hash
of HEAD.  Run 'git reset --hard "HEAD^^^"'.  Run 'git cherry-pick
HASH_OF_UPDATE_CHANGE'.  Run './update_depot_tools'.  Verify
depot_tools is updated to latest available revision + this change.

TEST=Windows command prompt + git clone: on Windows command prompt,
run Linux+git clone test, except use update_depot_tools.bat.  Verify
depot_tools is updated to latest available revision + this change.

TEST=Windows command prompt + git svn clone: on Windows command
prompt, run Linux+git svn clone test, except use
update_depot_tools.bat.  Verify depot_tools is updated to latest
available revision + this change.

TEST=Windows cygwin + git clone: on Windows command prompt,
run Linux+git clone test.  Verify depot_tools is updated to latest
available revision + this change.

TEST=Windows cygwin + git svn clone: on Windows command
prompt, run Linux+git svn clone test.  Verify depot_tools is updated
to latest available revision + this change.

Review URL: https://chromiumcodereview.appspot.com/10263004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@134732 0039d316-1c4b-4281-b951-d872f2087c98