log: follow file history across copies even with -rREV (BC) (issue4959)
Still it falls back to changelog path if glob patterns or missing paths
are specified. This will be changed later.
.. bc::
``log --follow -rREV FILE..`` now follows file history across copies and
renames.
log: resolve --follow thoroughly in getlogrevs()
This makes sense because --follow isn't really an option to filter revisions,
but an option to extend revisions to be filtered.
_fileancestors() is a minimal copy of revset._follow(). They are slightly
different in that which revision the matcher sees. _fileancestors() also
uses ctx.walk() instead of ctx.manifest().walk() to show a better warning
on bad match, which will be tested later.