Tighten refspec processing

Tighten refspec processing

This changes the pattern matching code to not store the required final
/ before the *, and then to require each side to be a valid ref (or
empty). In particular, any refspec that looks like it should be a
pattern but doesn't quite meet the requirements will be found to be
invalid as a fallback non-pattern.

This was cherry picked from commit ef00d15 (Tighten refspec processing,
2008-03-17), and two fix-up commits 46220ca (remote.c: Fix overtight
refspec validation, 2008-03-20) and 7d19da4 (refspec: allow colon-less
wildcard "refs/category/*", 2008-03-25) squashed in.

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>