Repoman doesn't recognise a git repository with a separate git dir, for example, cloned like this:
$ git clone --separate-git-dir gentoo.git git+ssh://firstname.lastname@example.org/repo/gentoo.git gentoo
When trying to do a commit, I get the following messages:
$ repoman commit
[WARNING] No VCS type detected, unable to sign the commit
RepoMan does a once-over of the neighborhood...
Note: use --include-dev (-d) to check dependencies for 'dev' profiles
RepoMan sez: "So, you want to play it safe. Good call."
RepoMan sez: "Doing nothing is not always good for QA."
(Didn't find any changed files...)
Note that in such a configuration, .git is not a directory but a regular file (containing a single line "gitdir: /path/to/gitdir.git").
Fixed in commit: https://gitweb.gentoo.org/proj/portage.git/commit/?id=6a26bdeafc4d113bb907f3accc971b61ba93d8c6