gpg-interface.c: obtain primary key fingerprint as well

Obtain the primary key fingerprint off VALIDSIG status message,
and expose it via %GP format.

Signed-off-by: Michał Górny <mgorny@gentoo.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/pretty-formats.txt

@@ -154,6 +154,8 @@ endif::git-rev-list[]
 - '%GS': show the name of the signer for a signed commit
 - '%GK': show the key used to sign a signed commit
 - '%GF': show the fingerprint of the key used to sign a signed commit
+- '%GP': show the fingerprint of the primary key whose subkey was used
+  to sign a signed commit
 - '%gD': reflog selector, e.g., `refs/stash@{1}` or
   `refs/stash@{2 minutes ago`}; the format follows the rules described
   for the `-g` option. The portion before the `@` is the refname as

gpg-interface.c

@@ -74,6 +74,7 @@ void signature_check_clear(struct signature_check *sigc)
 	FREE_AND_NULL(sigc->signer);
 	FREE_AND_NULL(sigc->key);
 	FREE_AND_NULL(sigc->fingerprint);
+	FREE_AND_NULL(sigc->primary_key_fingerprint);
 }
 
 /* An exclusive status -- only one of them can appear in output */
@@ -108,7 +109,7 @@ static void parse_gpg_output(struct signature_check *sigc)
 {
 	const char *buf = sigc->gpg_status;
 	const char *line, *next;
-	int i;
+	int i, j;
 	int seen_exclusive_status = 0;
 
 	/* Iterate over all lines */
@@ -147,6 +148,18 @@ static void parse_gpg_output(struct signature_check *sigc)
 					next = strchrnul(line, ' ');
 					free(sigc->fingerprint);
 					sigc->fingerprint = xmemdupz(line, next - line);
+
+					/* Skip interim fields */
+					for (j = 9; j > 0; j--) {
+						if (!*next)
+							break;
+						line = next + 1;
+						next = strchrnul(line, ' ');
+					}
+
+					next = strchrnul(line, '\n');
+					free(sigc->primary_key_fingerprint);
+					sigc->primary_key_fingerprint = xmemdupz(line, next - line);
 				}
 
 				break;
@@ -165,6 +178,7 @@ found_duplicate_status:
 	 */
 	sigc->result = 'E';
 	/* Clear partial data to avoid confusion */
+	FREE_AND_NULL(sigc->primary_key_fingerprint);
 	FREE_AND_NULL(sigc->fingerprint);
 	FREE_AND_NULL(sigc->signer);
 	FREE_AND_NULL(sigc->key);

gpg-interface.h

@@ -24,6 +24,7 @@ struct signature_check {
 	char *signer;
 	char *key;
 	char *fingerprint;
+	char *primary_key_fingerprint;
 };
 
 void signature_check_clear(struct signature_check *sigc);

pretty.c

@@ -1260,6 +1260,10 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 			if (c->signature_check.fingerprint)
 				strbuf_addstr(sb, c->signature_check.fingerprint);
 			break;
+		case 'P':
+			if (c->signature_check.primary_key_fingerprint)
+				strbuf_addstr(sb, c->signature_check.primary_key_fingerprint);
+			break;
 		default:
 			return 0;
 		}