diff: mark param1 and param2 as placeholders

Some l10n translators translated the parameters "files", "param1" and
"param2" in the following message:

    "synonym for --dirstat=files,param1,param2..."

Translating "param1" and "param2" is OK, but changing the parameter
"files" is wrong. The parameters that are not meant to be used verbatim
should be marked as placeholders, but the verbatim parameter not marked
as a placeholder should be left as is.

This change is a complement for commit 51e846e (doc: enforce
placeholders in documentation, 2023-12-25).

Signed-off-by: Jiang Xin <[email protected]>

Author: jiangxin

diff.c

@@ -5599,7 +5599,7 @@ struct option *add_diff_options(const struct option *opts,
 			       PARSE_OPT_NONEG | PARSE_OPT_NOARG,
 			       diff_opt_dirstat),
 		OPT_CALLBACK_F(0, "dirstat-by-file", options, N_("<param1,param2>..."),
-			       N_("synonym for --dirstat=files,param1,param2..."),
+			       N_("synonym for --dirstat=files,<param1>,<param2>..."),
 			       PARSE_OPT_NONEG | PARSE_OPT_OPTARG,
 			       diff_opt_dirstat),
 		OPT_BIT_F(0, "check", &options->output_format,