git-hooks/pre-commit: Allow merging deps2submodules changes from upstream
This also checks the 2nd parent during a merge, to allow commiting a
merge that contains newer versions of the submodules. In particular
merging chromium master into a local branch will now work.
TEST=git checkout -b test origin/master~200
git merge --no-ff --no-commit origin/master
git commit
Review URL: https://codereview.chromium.org/
192443006
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@259711 0039d316-1c4b-4281-b951-d872f2087c98