Change backlink method

Ensure every backlink is only set once.

Ensure every backlink points to a matching commit according to
cvs_match

previously branches could overwrite links from previous branches
with incorrect links.

See e.g. robotfindskitten configure.in 1.10
Tagging Redux

With correct backlinks, tagging looks deceptively easy.

Just find the latest tagged commit and then follow the backlink.

With all the history in this function, I might be missing a
subtlety.

However, if it works, tagging is now very fast.
Only check multiple gitspace assignement in debug
Fix FASTOUT issue with lines ending in @

Problem reported by Kevin L. Caswick, complete with test case.

Fix different behaviour between FASTOUT and not in lines ending
with a single @ but no line feed.
Reduce hash overhead.

hash_mix shows up in profiles on large repos. Use cached revdir
values instead.