diff: resurrect XDF_NEED_MINIMAL with --minimal

Earlier, 582aa00 (git diff too slow for a file, 2010-05-02)
unconditionally dropped XDF_NEED_MINIMAL option from the internal xdiff
invocation to help performance on pathological cases, while hinting that a
follow-up patch could reintroduce it with "--minimal" option from the
command line.

Make it so.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-options.txt

@@ -45,6 +45,10 @@ ifndef::git-format-patch[]
 	Synonym for `-p --raw`.
 endif::git-format-patch[]
 
+--minimal::
+	Spend extra time to make sure the smallest possible
+	diff is produced.
+
 --patience::
 	Generate a diff using the "patience diff" algorithm.