Make <identifier> lowercase as per CodingGuidelines

parse-options part

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

parse-options.h

@@ -141,7 +141,7 @@ struct option {
 	{ OPTION_NUMBER, 0, NULL, (v), NULL, (h), \
 	  PARSE_OPT_NOARG | PARSE_OPT_NONEG, (f) }
 #define OPT_FILENAME(s, l, v, h)    { OPTION_FILENAME, (s), (l), (v), \
-				       "FILE", (h) }
+				       "file", (h) }
 #define OPT_COLOR_FLAG(s, l, v, h) \
 	{ OPTION_CALLBACK, (s), (l), (v), "when", (h), PARSE_OPT_OPTARG, \
 		parse_opt_color_flag_cb, (intptr_t)"always" }