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.

log: extract function that processes log file patterns We'll need a matcher to compute revs followed from the given patterns.

log: resolve --follow with -rREV in cmdutil.getlogrevs() This also fixes alias expansion. Before, reverse() could be overridden by user alias. This isn't processed at _logrevs() as we'll need starting revisions to parse file patterns. See the subsequent patches for details.