diff: accept color.diff.context as a synonym for "plain"

The term "plain" is a bit ambiguous; let's allow the more
specific "context", but keep "plain" around for
compatibility.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -54,7 +54,7 @@ static char diff_colors[][COLOR_MAXLEN] = {
 
 static int parse_diff_color_slot(const char *var)
 {
-	if (!strcasecmp(var, "plain"))
+	if (!strcasecmp(var, "context") || !strcasecmp(var, "plain"))
 		return DIFF_PLAIN;
 	if (!strcasecmp(var, "meta"))
 		return DIFF_METAINFO;