reencode_string(): introduce and use same_encoding()

Callers of reencode_string() that re-encodes a string from one
encoding to another all used ad-hoc way to bypass the case where the
input and the output encodings are the same.  Some did strcmp(),
some did strcasecmp(), yet some others when converting to UTF-8 used
is_encoding_utf8().

Introduce same_encoding() helper function to make these callers use
the same logic.  Notably, is_encoding_utf8() has a work-around for
common misconfiguration to use "utf8" to name UTF-8 encoding, which
does not match "UTF-8" hence strcasecmp() would not consider the
same.  Make use of it in this helper function.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

notes.c

@@ -1221,7 +1221,7 @@ void format_note(struct notes_tree *t, const unsigned char *object_sha1,
 	}
 
 	if (output_encoding && *output_encoding &&
-			strcmp(utf8, output_encoding)) {
+	    !is_encoding_utf8(output_encoding)) {
 		char *reencoded = reencode_string(msg, output_encoding, utf8);
 		if (reencoded) {
 			free(msg);