Merge branch 'jk/color-diff-plain-is-context'

"color.diff.plain" was a misnomer; give it 'color.diff.context' as
a more logical synonym.

* jk/color-diff-plain-is-context:
  diff.h: rename DIFF_PLAIN color slot to DIFF_CONTEXT
  diff: accept color.diff.context as a synonym for "plain"

diff.h

@@ -181,7 +181,7 @@ struct diff_options {
 
 enum color_diff {
 	DIFF_RESET = 0,
-	DIFF_PLAIN = 1,
+	DIFF_CONTEXT = 1,
 	DIFF_METAINFO = 2,
 	DIFF_FRAGINFO = 3,
 	DIFF_FILE_OLD = 4,