blame: reserve mark column only if necessary

blame: reserve mark column only if necessary

git blame prepends commit hashes of boundary commits with "^", ignored
commits with "?" and unblamable commits with "*" and reserves one column
for them by extending the hash abbreviation, to avoid showing ambiguous
hashes.

This reserved column wastes precious screen space, which can be
especially irritating when using the option -b to blank out boundary
commit hashes and not ignoring any commits.  Reserve it only as needed,
i.e. if any of those cases are actually shown.

Pointed-out-by: Laszlo Ersek <laszlo.ersek@posteo.net>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>