fast-export: drop const when storing anonymized values

We store anonymized values as pointers to "const char *", since they are
conceptually const to callers who use them. But they are actually
allocated strings whose memory is owned by the struct.

The ownership mismatch hasn't been a big deal since we never free() them
(they are held until the program ends), but let's switch them to "char *"
in preparation for changing that.

Since most code only accesses them via anonymize_str(), it can continue
to narrow them to "const char *" in its return value.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/fast-export.c

@@ -110,7 +110,7 @@ static struct decoration idnums;
 static uint32_t last_idnum;
 struct anonymized_entry {
 	struct hashmap_entry hash;
-	const char *anon;
+	char *anon;
 	const char orig[FLEX_ARRAY];
 };