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.
 

diff.c

@@ -3385,6 +3385,10 @@ int diff_opt_parse(struct diff_options *options, const char **av, int ac)
 	}
 
 	/* xdiff options */
+	else if (!strcmp(arg, "--minimal"))
+		DIFF_XDL_SET(options, NEED_MINIMAL);
+	else if (!strcmp(arg, "--no-minimal"))
+		DIFF_XDL_CLR(options, NEED_MINIMAL);
 	else if (!strcmp(arg, "-w") || !strcmp(arg, "--ignore-all-space"))
 		DIFF_XDL_SET(options, IGNORE_WHITESPACE);
 	else if (!strcmp(arg, "-b") || !strcmp(arg, "--ignore-space-change"))