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>

pretty.c

@@ -504,7 +504,7 @@ char *logmsg_reencode(const struct commit *commit,
 		return NULL;
 	encoding = get_header(commit, "encoding");
 	use_encoding = encoding ? encoding : utf8;
-	if (!strcmp(use_encoding, output_encoding))
+	if (same_encoding(use_encoding, output_encoding))
 		if (encoding) /* we'll strip encoding header later */
 			out = xstrdup(commit->buffer);
 		else