gpg-interface: expand gpg.program as a path

This allows using a custom gpg program under the user's home directory
by specifying a path starting with '~'

[gpg]
        program = "~/.local/bin/mygpg"

Signed-off-by: Jonas Brandstötter <jonas.brandstoetter@gmx.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/gpg.adoc

@@ -1,5 +1,5 @@
 gpg.program::
-	Use this custom program instead of "`gpg`" found on `$PATH` when
+	Pathname of the program to use instead of "`gpg`" 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 $signature - <$file`" is run, and the

gpg-interface.c

@@ -783,7 +783,7 @@ static int git_gpg_config(const char *var, const char *value,
 
 	if (fmtname) {
 		fmt = get_format_by_name(fmtname);
-		return git_config_string((char **) &fmt->program, var, value);
+		return git_config_pathname((char **) &fmt->program, var, value);
 	}
 
 	return 0;