revlog: rename _chunkraw to _getsegmentforrevs()
This completes our rename of internal revlog methods to
distinguish between low-level raw revlog data "segments" and
higher-level, per-revision "chunks."
perf.py has been updated to consult both names so it will work
against older Mercurial versions.
#require docutils gettext
$ $TESTDIR/check-gendoc da
checking for parse errors