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>

builtin/revert.c

@@ -179,7 +179,7 @@ static int run_sequencer(int argc, const char **argv, const char *prefix,
 
 	/* Check for incompatible command line arguments */
 	if (cmd) {
-		char *this_operation;
+		const char *this_operation;
 		if (cmd == 'q')
 			this_operation = "--quit";
 		else if (cmd == 'c')