global: improve const correctness when assigning string constants

We're about to enable `-Wwrite-strings`, which changes the type of
string constants to `const char[]`. Fix various sites where we assign
such constants to non-const variables.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/gpg-interface.c b/gpg-interface.c
index 5193223..71a9382 100644
--- a/gpg-interface.c
+++ b/gpg-interface.c
@@ -727,7 +727,7 @@ static int git_gpg_config(const char *var, const char *value,
 			  void *cb UNUSED)
 {
 	struct gpg_format *fmt = NULL;
-	char *fmtname = NULL;
+	const char *fmtname = NULL;
 	char *trust;
 	int ret;