Optimize color_parse_mem

Optimize color_parse_mem

Commit 5ef8d77a implemented color_parse_mem, a function for
parsing colors from a non-NUL-terminated string, by simply
allocating a new NUL-terminated string and calling
color_parse. This had a small but measurable speed impact on
a user format that used the advanced color parsing. E.g.,

  # uses quick parsing
  $ time ./git log --pretty=tformat:'%Credfoo%Creset' >/dev/null
  real    0m0.673s
  user    0m0.652s
  sys     0m0.016s

  # uses color_parse_mem
  $ time ./git log --pretty=tformat:'%C(red)foo%C(reset)' >/dev/null
  real    0m0.692s
  user    0m0.660s
  sys     0m0.032s

This patch implements color_parse_mem as the primary
function, with color_parse as a wrapper for strings. This
gives comparable timings to the first case above.

Original patch by René. Commit message and debugging by Jeff
King.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>