deps/Makefile: fix checkout logic for branches

Checkout origin/$branch instead of $branch. Otherwise git will just use
the local version of branch and stay at the same commit.

Note that Ia846863eff1734eca469257b9a8d5e653a45e835 changes the lines
which still have "master" to the commit. Fixing this is still useful
however, if a branch name is used in local testing.

Change-Id: If6bd440598d375a9947cb3a4fd88ae5809756e0e
1 file changed