revlog: add a fast path for revision(raw=False)
If cache hit and flags are empty, no flag processor runs and "text" equals
to "rawtext". So we check flags, and return rawtext.
This resolves performance issue introduced by a previous patch.
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a