A list of already-authenticated commits is kept in
~/.cache/guix/authentication. This speeds up subsequent "make
authenticate" invocations.
* build-aux/git-authenticate.scm (authenticated-commit-cache-file)
(previously-authenticated-commits, cache-authenticated-commit): New
procedures.
(git-authenticate): Define 'authenticated-commits' and pass it as a
third argument to 'commit-difference'. Add call to
'cache-authenticated-commit'. Don't display signing stats when STATS is
null.
