Bug report: --invert-grep and --author seems to be incompatible.

Hello,

$ git version
git version 2.5.5

Tested on a Fedora 23.

You can combine `git log --author=someone --grep=something` to have
all commits by "someone" where "something" can be grepped. If I want
all commits by someone where "something" is not grepped, I would
expect this command to return it:

git log --author=someone --invert-grep --grep=something

But it does not work. Actually it looks like it just returns
everything (as though I had done a simple `git log`). So there seems
to be a problem here.
Thanks!

Jehan

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