#993 fix: wrong refs 'HEAD' exception

[dongs0104]

When git HEAD refs is broken, it raise BadName exception.
so submodules logic which is using a rev_parse function to HEAD get Exception, so handle BadName to return empty list

b303cb0

GitPython/git/diff.py

Lines 281 to 287 in b303cb0

	# Determine whether this diff references a submodule, if it does then
	# we need to overwrite "repo" to the corresponding submodule's repo instead
	if repo and a_rawpath:
	for submodule in repo.submodules:
	if submodule.path == a_rawpath.decode("utf-8"):
	repo = submodule.module()
	break

[Byron]

Thanks a lot for your contribution :)! It's much appreciated.

CC: #993 , where diff is broken due to a very similar issue.