Currently layman has the path to git hardcoded in layman/overlays/git.py. On systems with the git binary not located at /usr/bin/git (ie: installed in /opt/git from source) layman is unable to process any git-based overlays.
Steps to Reproduce:
1. Move /usr/bin/git to /root/bin/git
2. Add /root/bin to your PATH environment variable
3. Test git works by executing `git --version`
4. Use layman to update or add a git-based overlay
layman complains that the git binary is missing and suggests I install git.
layman should happily process my request without complaints.
Out of curiosity: Why do you install Git bypassing Portage? Customizing and ebuild and making Portage eat that is easy. If you're developing Git itself that may be another story, okay.
It seems you're not shy of working with source code. Do you have time to make us a patch?
With the current code in Git this is now supported.
You can emerge app-portage/layman-9999 (Layman live ebuild) to check it out.
Add "git_command : git" to /etc/layman/layman.cfg to activate it, default is still /usr/bin/git.