git-log: added --invert-grep option

Implements a inverted match for "git log", like in the case of
"git grep -v", which is useful from time to time to e.g. filter
FIXUP message out of "git log".

Internally, a new bol 'global_invert' has been introduces as
revs->grep_filter.invert inverts the match line-wise, which cannot
work as i.e. empty line always not match the pattern given.

Signed-off-by: Christoph Junghans <junghans@lanl.gov>