Custom git GEOMETRY_RPROMPT spews to stderr

[duncanbeevers]

Describe the bug
The meta-function geometry_git guards against non-git directories.
Customizing GEOMETRY_RPROMPT with a limited set of geometry_git_* functions doesn't benefit from this guard so the prompt spews to stderr in non-git directories.

Including the complete set of geometry_git_* functions without the preamble guard provided by geometry_git is especially noisy.

To Reproduce
Steps to reproduce the behavior:

1. Customize .zshrc with export GEOMETRY_RPROMPT=(geometry_git_rebase)
2. Open shell, navigate to non-git directory
3. See error

Expected behavior
No errors, no stderr output

Screenshots
A single geometry_git_* function referenced

All default geometry_git_* functions referenced

Desktop (please complete the following information):

• OS: macOS 10.15.5 (19F101)