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.h

@@ -3,6 +3,7 @@
 
 typedef unsigned int ucs_char_t;  /* assuming 32bit int */
 
+size_t display_mode_esc_sequence_len(const char *s);
 int utf8_width(const char **start, size_t *remainder_p);
 int utf8_strnwidth(const char *string, int len, int skip_ansi);
 int utf8_strwidth(const char *string);