In --follow mode, we require exactly one pathspec. We check this
condition in two places:
- in diff_setup_done(), we complain if --follow is used with an
inapropriate pathspec
- in git-log's revision "tweak" function, we enable log.follow only if
the pathspec allows it
The duplication isn't a big deal right now, since the logic is so
simple. But in preparation for it becoming more complex, let's pull it
into a shared function.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>