rhg: desambiguate status without decompressing filelog if possible
When status is unsure based on `stat()` and the dirstate if a file is clean
or modified, we need to compare it against the filelog.
This comparison can skip looking at contents if the lengths differ.
This changeset optimize this further to deduce what we can about the length
if the filelog without decompressing it or resolving deltas.
Differential Revision: https://phab.mercurial-scm.org/D11965
$ hg init
$ "$PYTHON" -c 'open("a", "wb").write(b"confuse str.splitlines\nembedded\rnewline\n")'
$ hg ci -Ama -d '1 0'
adding a
$ echo clean diff >> a
$ hg ci -mb -d '2 0'
$ hg diff -r0 -r1
diff -r 107ba6f817b5 -r 310ce7989cdc a
--- a/a Thu Jan 01 00:00:01 1970 +0000
+++ b/a Thu Jan 01 00:00:02 1970 +0000
@@ -1,2 +1,3 @@
confuse str.splitlines
embedded\r (no-eol) (esc)
newline
+clean diff