pretty: support padding placeholders, %< %> and %><

pretty: support padding placeholders, %< %> and %><

Either %<, %> or %>< standing before a placeholder specifies how many
columns (at least as the placeholder can exceed it) it takes. Each
differs on how spaces are padded:

  %< pads on the right (aka left alignment)
  %> pads on the left (aka right alignment)
  %>< pads both ways equally (aka centered)

The (<N>) follows them, e.g. `%<(100)', to specify the number of
columns the next placeholder takes.

However, if '|' stands before (<N>), e.g. `%>|(100)', then the number
of columns is calculated so that it reaches the Nth column on screen.

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