if when execute command "git rev-parse --show-toplevel" in a directory
that is not a subdirectory of a git repo, CheckCallAndFilter will not
throw exception, so the top_dir may be updated with a invalid path.

BUG=633971
Signed-off-by: hanpfei <hanpfei@gmail.com>
Review-Url: https://codereview.chromium.org/2200193003