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/diff.c b/diff.c
index e70301d..ffd867e 100644
--- a/diff.c
+++ b/diff.c
@@ -3764,7 +3764,7 @@ static void builtin_diff(const char *name_a,
 	return;
 }
 
-static char *get_compact_summary(const struct diff_filepair *p, int is_renamed)
+static const char *get_compact_summary(const struct diff_filepair *p, int is_renamed)
 {
 	if (!is_renamed) {
 		if (p->status == DIFF_STATUS_ADDED) {
@@ -4076,7 +4076,7 @@ static int reuse_worktree_file(struct index_state *istate,
 static int diff_populate_gitlink(struct diff_filespec *s, int size_only)
 {
 	struct strbuf buf = STRBUF_INIT;
-	char *dirty = "";
+	const char *dirty = "";
 
 	/* Are we looking at the work tree? */
 	if (s->dirty_submodule)