Allow the Git repository path to be specified. #1467

In support of git-bug's use as a tool (similar to what's described in #828), this issue promotes composability by allowing the calling program to specify the Git repository's path. This should be accomplished in the same manner as using the -C flag with the git command. Separate handling of the Git directory and working tree, similar to using the Git --git-dir and --work-tree flags are out-of-scope (at least for the time being.)

For reference, the relevant portion of the documentation for Git's -C flag is:

Run as if git was started in instead of the current working directory. When multiple -C options are
given, each subsequent non-absolute -C is interpreted relative to the preceding -C . If
is present but empty, e.g. -C "", then the current working directory is left unchanged.