Allow git to run on an other user repository

Allow git to run on an other user repository

Reported by Anton:

"Recent versions of libgit2 broke stagit for me due to the added opt-out
GIT_OPT_SET_OWNER_VALIDATION configuration knob. My repositories are owned by
root:vcs and I run stagit as another user which happens to be in vcs group but
not the owner of the repository. Disabling the validation makes stagit work as
expected again."

Some notes:

When using regular git it also provides a knob. This is due to a security
concern in some cases, which is not applicable to stagit.

	git log somerepo

	fatal: unsafe repository ('somerepo' is owned by someone else)
	To add an exception for this directory, call:

	        git config --global --add safe.directory somerepo

See also / related:
- https://github.blog/2022-04-12-git-security-vulnerability-announced/