color: fix have_fg not re assign to true

In method _parse the value of this variable 'have_fg ' is always
False, Maybe reassign it to True is lost.
I guess the author’s original intention was:
if set some value in gitconfig file(for ex: text = black red ul),
the first is bg color, the second is fg color, and the last one is attr.



Change-Id: I372698fe625db4c1fdaa94ea7f193a80a850ecb9
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/425997
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Bright Ma <mmh1989@foxmail.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>