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>

Documentation/config.txt

@@ -870,7 +870,8 @@ command line with the `--color[=<when>]` option.
 color.diff.<slot>::
 	Use customized color for diff colorization.  `<slot>` specifies
 	which part of the patch to use the specified color, and is one
-	of `plain` (context text), `meta` (metainformation), `frag`
+	of `context` (context text - `plain` is a historical synonym),
+	`meta` (metainformation), `frag`
 	(hunk header), 'func' (function in hunk header), `old` (removed lines),
 	`new` (added lines), `commit` (commit headers), or `whitespace`
 	(highlighting whitespace errors). The values of these variables may be

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;