Patches removing lines ending in "by:" will cause false-positive
warnings about non-standard / malformed signatures. Only looking for
malformed signatures in the commit message avoids this.

Given this patch:
> From: Aren Moynihan <aren@peacevolution.org>
> Date: Wed, 2 Apr 2025 16:34:53 -0400
> Subject: [PATCH] Test commit
>
> This is a test commit
>
> Signed-off-by: Aren Moynihan <aren@peacevolution.org>
> ---
>  test-file.c | 1 -
>  1 file changed, 1 deletion(-)
>
> diff --git a/test-file.c b/test-file.c
> index 1527673..71c7e44 100644
> --- a/test-file.c
> +++ b/test-file.c
> @@ -1,3 +1,2 @@
>  int foo() {
> -err_standby:
>  }

Before:
> WARNING: Non-standard signature: -err_standby:
> #19: FILE: test-file.c:1:
> -err_standby:
>
> WARNING: Use a single space after -err_standby:
> #19: FILE: test-file.c:1:
> -err_standby:
>
> ERROR: Unrecognized email address: ''
> #19: FILE: test-file.c:1:
> -err_standby:
>
> total: 1 errors, 2 warnings, 3 lines checked

After:
> total: 0 errors, 0 warnings, 3 lines checked

A real world example of this can be found here:
https://lore.kernel.org/lkml/20250208211325.992280-4-aren@peacevolution.org/

Signed-off-by: Aren Moynihan <aren@peacevolution.org>
---
 scripts/checkpatch.pl | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
index 784912f570e9d..a7bb6df8510d1 100755
--- a/scripts/checkpatch.pl
+++ b/scripts/checkpatch.pl
@@ -3014,7 +3014,7 @@ sub process {
 		}
 
 # Check signature styles
-		if (!$in_header_lines &&
+		if ($in_commit_log &&
 		    $line =~ /^(\s*)([a-z0-9_-]+by:|$signature_tags)(\s*)(.*)/i) {
 			my $space_before = $1;
 			my $sign_off = $2;
-- 
2.49.0