allow running from any directory

Currently `git deps` must be run from the root (top directory) of the git repo. It would be better if it could be run from any subdirectory, or even from outside the repo being examined. See also #22.

Start by reading the CLI implementation to find where the repository root and target repository are determined, then review issue #22 for related context. Update the command so invocation from a subdirectory or outside the examined repository works, and verify both invocation locations produce the expected dependency analysis.

Written by the indexing model from the issue text.