rewriteutil: handle dropped commits when updating description hashes
In looking to leverage this with the absorb extension, the old -> new mapping
there allows the new value to be None. We could filter that out and not pass it
to this method, but it seems worth a message to the user. (I wonder if these
should be an info or warning, because it's unlikely people are using `-v`
regularly.)
Differential Revision: https://phab.mercurial-scm.org/D9181
$ 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