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.
$ hg init
$ echo a > a
$ hg ci -Am t
adding a
$ hg mv a b
$ hg ci -Am t1
$ hg debugrename b
b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3
$ hg mv b a
$ hg ci -Am t2
$ hg debugrename a
a renamed from b:37d9b5d994eab34eda9c16b195ace52c7b129980
$ hg debugrename --rev 1 b
b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3