pretty: support %>> that steal trailing spaces

This is pretty useful in `%<(100)%s%Cred%>(20)% an' where %s does not
use up all 100 columns and %an needs more than 20 columns. By
replacing %>(20) with %>>(20), %an can steal spaces from %s.

%>> understands escape sequences, so %Cred does not stop it from
stealing spaces in %<(100).

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

utf8.c

@@ -9,7 +9,7 @@ struct interval {
   int last;
 };
 
-static size_t display_mode_esc_sequence_len(const char *s)
+size_t display_mode_esc_sequence_len(const char *s)
 {
 	const char *p = s;
 	if (*p++ != '\033')