Merge SHA issue with Git 1.7.10

[gracinet]

With git version 1.7.10.4 (the one shipped with Debian 7), the unit tests don't pass.

From /tmp/tmpexTpCZtest_oerp_recipe_vcs/src/src-repo
 * [new branch]      branch1    -> origin/branch1
 * [new branch]      branch2    -> origin/branch2
fatal: Couldn't find remote ref 84cf26e6c92f26c748c170febfc4beee59499814
fatal: The remote end hung up unexpectedly
> /usr/lib/python2.7/subprocess.py(511)check_call()
-> raise CalledProcessError(retcode, cmd)
(Pdb) pp cmd
['git',
 'pull',
 '--no-edit',
 '/tmp/tmpexTpCZtest_oerp_recipe_vcs/src/src-repo',
 '84cf26e6c92f26c748c170febfc4beee59499814']

I don't think pull on SHA has been added to Git since that version, so it's probably some kind of side effect.