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>

utf8.c

@@ -423,6 +423,13 @@ int is_encoding_utf8(const char *name)
 	return 0;
 }
 
+int same_encoding(const char *src, const char *dst)
+{
+	if (is_encoding_utf8(src) && is_encoding_utf8(dst))
+		return 1;
+	return !strcasecmp(src, dst);
+}
+
 /*
  * Given a buffer and its encoding, return it re-encoded
  * with iconv.  If the conversion fails, returns NULL.