WIP: plugins/sources/git.py: Cope with rename returning error EEXIST

Description

POSIX rename can set errno to ENOTEMPTY or EEXIST when the destination dir already has content (http://pubs.opengroup.org/onlinepubs/009695399/functions/rename.html), however the git plugin only copes with ENOTEMPTY (in 1.2.4)

In our project we have many elements referencing the same git repo, so this has causes some issues when building for the first time.

I've noticed this has already been fixed on master in utils.move_atomic - this is just a backport of that functionality to git plugin in 1.2.

Changes proposed in this merge request:

• Check for EEXIST on os.rename failure in the git plugin.

This merge request, when approved, will close:

---