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>

pretty.c

@@ -1325,7 +1325,7 @@ int format_set_trailers_options(struct process_trailer_options *opts,
 static size_t parse_describe_args(const char *start, struct strvec *args)
 {
 	struct {
-		char *name;
+		const char *name;
 		enum {
 			DESCRIBE_ARG_BOOL,
 			DESCRIBE_ARG_INTEGER,