[PATCH] Add --diff-filter= output restriction to diff-* family.
This is a halfway between debugging aid and a helper to write an
ultra-smart merge scripts. The new option takes a string that
consists of a list of "status" letters, and limits the diff
output to only those classes of changes, with two exceptions:
- A broken pair (aka "complete rewrite"), does not match D
(deleted) or N (created). Use B to look for them.
- The letter "A" in the diff-filter string does not match
anything itself, but causes the entire diff that contains
selected patches to be output (this behaviour is similar to
that of --pickaxe-all for the -S option).