diff: add --inverse option Most of the time, one can reverse a diff by swapping the revisions passed with -r but it happens that if you use the global -R, and diff against the tip of the current repo, you can't swap the revisions. One use-case for that is reviewing changes from a bundle before unbundling. One could also pipe the output of `hg diff` to a command line filter that reverses the diff, but that would remove the benefit from color diffs. Therefore, having an option in `hg diff` to reverse a diff is a good thing. The option flag selection was tricky. GNU patch uses -R/--reverse but -R is already used as a global option and --reverse would make --rev ambiguous.

adding a
diff -r 2855cdcfcbb7 -r 8e1805a3cf6e a
--- a/a	Thu Jan 01 00:00:00 1970 +0000
+++ b/a	Thu Jan 01 00:00:00 1970 +0000
@@ -1,3 +1,3 @@
-d
-e
-f
+a
+b
+c
diff -r 2855cdcfcbb7 a
--- a/a
+++ b/a
@@ -1,5 +1,3 @@
 d
 e
 f
-g
-h