log: read gpg settings for signed commit verification

"show --show-signature" and "log --show-signature" do not read the
gpg.program setting from git config, even though, commit signing,
tag signing, and tag verification honor it.

Signed-off-by: Jacob Sarvis <jsarvis@openspan.com>
Signed-off-by: Hans Brigman <hbrigman@openspan.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/log.c

@@ -23,6 +23,7 @@
 #include "streaming.h"
 #include "version.h"
 #include "mailmap.h"
+#include "gpg-interface.h"
 
 /* Set a default date-time format for git log ("log.date" config variable) */
 static const char *default_date_mode = NULL;
@@ -367,6 +368,8 @@ static int git_log_config(const char *var, const char *value, void *cb)
 
 	if (grep_config(var, value, cb) < 0)
 		return -1;
+	if (git_gpg_config(var, value, cb) < 0)
+		return -1;
 	return git_diff_ui_config(var, value, cb);
 }