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: Junio C Hamano <junkio@cox.net>

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