rebase -i: finishing touches to --reset-author-date

Clarify the way the `--reset-author-date` option is described,
and mark its usage string translatable.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-rebase.txt

@@ -392,8 +392,10 @@ See also INCOMPATIBLE OPTIONS below.
 
 --ignore-date::
 --reset-author-date::
-	Instead of using the given author date, reset it to the
-	current time. This implies --force-rebase.
+	By default, the author date of the original commit is used
+	as the author date for the resulting commit.  This option
+	tells Git to use the current timestamp instead and implies
+	`--force-rebase`.
 +
 See also INCOMPATIBLE OPTIONS below.
 

builtin/rebase.c

@@ -1437,9 +1437,9 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 			 &options.committer_date_is_author_date,
 			 N_("make committer date match author date")),
 		OPT_BOOL(0, "reset-author-date", &options.ignore_date,
-			 "ignore author date and use current date"),
-		OPT_BOOL(0, "ignore-date", &options.ignore_date,
-			 "ignore author date and use current date"),
+			 N_("ignore author date and use current date")),
+		OPT_HIDDEN_BOOL(0, "ignore-date", &options.ignore_date,
+				N_("synonym of --reset-author-date")),
 		OPT_PASSTHRU_ARGV('C', NULL, &options.git_am_opts, N_("n"),
 				  N_("passed to 'git apply'"), 0),
 		OPT_BOOL(0, "ignore-whitespace", &options.ignore_whitespace,