commit-tree: cope with different ways "utf-8" can be spelled.

People can spell config.commitencoding differently from what we
internally have ("utf-8") to mean UTF-8.  Try to accept them and
treat them equally.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

builtin-commit-tree.c

@@ -119,8 +119,7 @@ int cmd_commit_tree(int argc, const char **argv, const char *prefix)
 	}
 
 	/* Not having i18n.commitencoding is the same as having utf-8 */
-	encoding_is_utf8 = (!git_commit_encoding ||
-			    !strcmp(git_commit_encoding, "utf-8"));
+	encoding_is_utf8 = is_encoding_utf8(git_commit_encoding);
 
 	init_buffer(&buffer, &size);
 	add_buffer(&buffer, &size, "tree %s\n", sha1_to_hex(tree_sha1));

utf8.c

@@ -277,6 +277,15 @@ void print_wrapped_text(const char *text, int indent, int indent2, int width)
 	}
 }
 
+int is_encoding_utf8(const char *name)
+{
+	if (!name)
+		return 1;
+	if (!strcasecmp(name, "utf-8") || !strcasecmp(name, "utf8"))
+		return 1;
+	return 0;
+}
+
 /*
  * Given a buffer and its encoding, return it re-encoded
  * with iconv.  If the conversion fails, returns NULL.

utf8.h

@@ -3,6 +3,8 @@
 
 int utf8_width(const char **start);
 int is_utf8(const char *text);
+int is_encoding_utf8(const char *name);
+
 void print_wrapped_text(const char *text, int indent, int indent2, int len);
 
 #ifndef NO_ICONV