log: speed up single file log with hidden revs (issue4747) On repos with lots of heads, the filelog() code could spend several minutes decompressing manifests. This change instead tries to efficiently scan the changelog for candidates and decompress as few manifests as possible. This is a regression introduced in 3.3 by the linkrev adjustment code. Prior to that, filelog was nearly instant. For the repo in the bug report, this improves time of a simple log command from ~3 minutes to ~.5 seconds, a 360x speedup. For the main Mercurial repo, a log of commands.py slows down from 1.14s to 1.45s, a 27% slowdown. This is still faster than the file() revset, which takes 2.1 seconds.