diff: avoid misleading statement about -l option

In commit 6623a52 (doc: clarify documentation for rename/copy
limits, 2021-07-15), the wording around rename limit options and config
variables were updated to point out that only the quadratic portion of
rename detection (or "exhaustive portion of rename/copy detection" as
used in that commit) was limited by these options, because exact rename
detection and basename-guided rename detection (which both run in time
linear in the number of files) still run before this limit is checked.

However, the short help message wasn't updated at the time; update it
too.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: newren

diff.c

@@ -6186,7 +6186,7 @@ struct option *add_diff_options(const struct option *opts,
 			       N_("continue listing the history of a file beyond renames"),
 			       PARSE_OPT_NOARG, diff_opt_follow),
 		OPT_INTEGER('l', NULL, &options->rename_limit,
-			    N_("prevent rename/copy detection if the number of rename/copy targets exceeds given limit")),
+			    N_("limit to cheap rename/copy detection if the number of rename/copy targets exceeds this value")),
 
 		OPT_GROUP(N_("Diff algorithm options")),
 		OPT_CALLBACK_F(0, "minimal", options, NULL,