git-mfc: Add an ignore-list feature

Commit hashes listed in ~/.git-mfc-ignore are not listed in output of
git-mfc --dangling or --pending. This is handy for silencing output
about commits that are tagged for MFC or as fixing another commit, but
which were not MFCed for some reason or other.

Requested by: des
Reviewed by: des
Differential Revision: https://reviews.freebsd.org/D58161