Alain Takoudjou authored Jun 29, 2020 and Kirill Smelkov committed Jul 02, 2020

If there is a folder in git repositories which ends with .git and which
is not a git repository, git-backup will fail because it will try to use
it as a git repository.

This patch just checks if it's really a git repo before continue inside.

[kirr: Reworked the patch a bit]
/reviewed-on: kirr/git-backup!8