Merge branch 'jk/refspec-parse-wildcard'

Allow an asterisk as a substring (as opposed to the entirety) of
a path component for both side of a refspec, e.g.
"refs/heads/o*:refs/remotes/heads/i*".

* jk/refspec-parse-wildcard:
refs: loosen restriction on wildcard "*" refspecs
refs: cleanup comments regarding check_refname_component()