diff: Introduce --diff-algorithm command line option

Since command line options have higher priority than config file
variables and taking previous commit into account, we need a way
how to specify myers algorithm on command line. However,
inventing `--myers` is not the right answer. We need far more
general option, and that is `--diff-algorithm`.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/merge-recursive.c b/merge-recursive.c
index 33ba5dc..ea9dbd3 100644
--- a/merge-recursive.c
+++ b/merge-recursive.c
@@ -2068,6 +2068,15 @@
 		o->xdl_opts = DIFF_WITH_ALG(o, PATIENCE_DIFF);
 	else if (!strcmp(s, "histogram"))
 		o->xdl_opts = DIFF_WITH_ALG(o, HISTOGRAM_DIFF);
+	else if (!strcmp(s, "diff-algorithm=")) {
+		long value = parse_algorithm_value(s+15);
+		if (value < 0)
+			return -1;
+		/* clear out previous settings */
+		DIFF_XDL_CLR(o, NEED_MINIMAL);
+		o->xdl_opts &= ~XDF_DIFF_ALGORITHM_MASK;
+		o->xdl_opts |= value;
+	}
 	else if (!strcmp(s, "ignore-space-change"))
 		o->xdl_opts |= XDF_IGNORE_WHITESPACE_CHANGE;
 	else if (!strcmp(s, "ignore-all-space"))