ssh signing: better error message when key not in agent

When signing a commit with a SSH key, with the private key missing from
ssh-agent, a confusing error message is produced:

    error: Load key
    "/var/folders/t5/cscwwl_n3n1_8_5j_00x_3t40000gn/T//.git_signing_key_tmpkArSj7":
    invalid format? fatal: failed to write commit object

The temporary file .git_signing_key_tmpkArSj7 created by git contains a
valid *public* key.  The error message comes from `ssh-keygen -Y sign' and
is caused by a fallback mechanism in ssh-keygen whereby it tries to
interpret .git_signing_key_tmpkArSj7 as a *private* key if it can't find in
the agent [1].  A fix is scheduled to be released in OpenSSH 9.1. All that
needs to be done is to pass an additional backward-compatible option -U to
'ssh-keygen -Y sign' call.  With '-U', ssh-keygen always interprets the file
as public key and expects to find the private key in the agent.

As a result, when the private key is missing from the agent, a more accurate
error message gets produced:

    error: Couldn't find key in agent

[1] https://bugzilla.mindrot.org/show_bug.cgi?id=3429

Signed-off-by: Adam Szkoda <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: adaszko

gpg-interface.c

@@ -998,13 +998,15 @@ static int sign_buffer_ssh(struct strbuf *buffer, struct strbuf *signature,
 	char *ssh_signing_key_file = NULL;
 	struct strbuf ssh_signature_filename = STRBUF_INIT;
 	const char *literal_key = NULL;
+	int literal_ssh_key = 0;
 
 	if (!signing_key || signing_key[0] == '\0')
 		return error(
 			_("user.signingKey needs to be set for ssh signing"));
 
 	if (is_literal_ssh_key(signing_key, &literal_key)) {
 		/* A literal ssh key */
+		literal_ssh_key = 1;
 		key_file = mks_tempfile_t(".git_signing_key_tmpXXXXXX");
 		if (!key_file)
 			return error_errno(
@@ -1039,8 +1041,10 @@ static int sign_buffer_ssh(struct strbuf *buffer, struct strbuf *signature,
 		     "-Y", "sign",
 		     "-n", "git",
 		     "-f", ssh_signing_key_file,
-		     buffer_file->filename.buf,
 		     NULL);
+	if (literal_ssh_key)
+		strvec_push(&signer.args, "-U");
+	strvec_push(&signer.args, buffer_file->filename.buf);
 
 	sigchain_push(SIGPIPE, SIG_IGN);
 	ret = pipe_command(&signer, NULL, 0, NULL, 0, &signer_stderr, 0);