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>

builtin/mailinfo.c

@@ -507,7 +507,7 @@ static void convert_to_utf8(struct strbuf *line, const char *charset)
 			return;
 	}
 
-	if (!strcasecmp(metainfo_charset, charset))
+	if (same_encoding(metainfo_charset, charset))
 		return;
 	out = reencode_string(line->buf, metainfo_charset, charset);
 	if (!out)

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);

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

sequencer.c

@@ -58,7 +58,7 @@ static int get_message(struct commit *commit, struct commit_message *out)
 
 	out->reencoded_message = NULL;
 	out->message = commit->buffer;
-	if (strcmp(encoding, git_commit_encoding))
+	if (same_encoding(encoding, git_commit_encoding))
 		out->reencoded_message = reencode_string(commit->buffer,
 					git_commit_encoding, encoding);
 	if (out->reencoded_message)

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.

utf8.h

@@ -7,6 +7,7 @@ int utf8_width(const char **start, size_t *remainder_p);
 int utf8_strwidth(const char *string);
 int is_utf8(const char *text);
 int is_encoding_utf8(const char *name);
+int same_encoding(const char *, const char *);
 
 int strbuf_add_wrapped_text(struct strbuf *buf,
 		const char *text, int indent, int indent2, int width);