gpg-interface: allow use of a custom GPG binary

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

Documentation/config.txt

@@ -1094,6 +1094,17 @@ grep.lineNumber::
 grep.extendedRegexp::
 	If set to true, enable '--extended-regexp' option by default.
 
+gpg.program::
+	Use this custom program instead of "gpg" found on $PATH when
+	making or verifying a PGP signature. The program must support the
+	same command line interface as GPG, namely, to verify a detached
+	signature, "gpg --verify $file - <$signature" is run, and the
+	program is expected to signal a good signature by exiting with
+	code 0, and to generate an ascii-armored detached signature, the
+	standard input of "gpg -bsau $key" is fed with the contents to be
+	signed, and the program is expected to send the result to its
+	standard output.
+
 gui.commitmsgwidth::
 	Defines how wide the commit message window is in the
 	linkgit:git-gui[1]. "75" is the default.

Documentation/git-tag.txt

@@ -38,7 +38,9 @@ created (i.e. a lightweight tag).
 A GnuPG signed tag object will be created when `-s` or `-u
 <key-id>` is used.  When `-u <key-id>` is not used, the
 committer identity for the current user is used to find the
-GnuPG key for signing.
+GnuPG key for signing. 	The configuration variable `gpg.program`
+is used to specify custom GnuPG binary.
+
 
 OPTIONS
 -------
@@ -48,11 +50,11 @@ OPTIONS
 
 -s::
 --sign::
-	Make a GPG-signed tag, using the default e-mail address's key
+	Make a GPG-signed tag, using the default e-mail address's key.
 
 -u <key-id>::
 --local-user=<key-id>::
-	Make a GPG-signed tag, using the given key
+	Make a GPG-signed tag, using the given key.
 
 -f::
 --force::

gpg-interface.c

@@ -5,6 +5,7 @@
 #include "sigchain.h"
 
 static char *configured_signing_key;
+static const char *gpg_program = "gpg";
 
 void set_signing_key(const char *key)
 {
@@ -15,9 +16,12 @@ void set_signing_key(const char *key)
 int git_gpg_config(const char *var, const char *value, void *cb)
 {
 	if (!strcmp(var, "user.signingkey")) {
+		set_signing_key(value);
+	}
+	if (!strcmp(var, "gpg.program")) {
 		if (!value)
 			return config_error_nonbool(var);
-		set_signing_key(value);
+		gpg_program = xstrdup(value);
 	}
 	return 0;
 }
@@ -46,7 +50,7 @@ int sign_buffer(struct strbuf *buffer, struct strbuf *signature, const char *sig
 	gpg.argv = args;
 	gpg.in = -1;
 	gpg.out = -1;
-	args[0] = "gpg";
+	args[0] = gpg_program;
 	args[1] = "-bsau";
 	args[2] = signing_key;
 	args[3] = NULL;
@@ -101,10 +105,11 @@ int verify_signed_buffer(const char *payload, size_t payload_size,
 			 struct strbuf *gpg_output)
 {
 	struct child_process gpg;
-	const char *args_gpg[] = {"gpg", "--verify", "FILE", "-", NULL};
+	const char *args_gpg[] = {NULL, "--verify", "FILE", "-", NULL};
 	char path[PATH_MAX];
 	int fd, ret;
 
+	args_gpg[0] = gpg_program;
 	fd = git_mkstemp(path, PATH_MAX, ".git_vtag_tmpXXXXXX");
 	if (fd < 0)
 		return error("could not create temporary file '%s': %s",