Yesterday I discovered the useful vc-log-outgoing command.  It isn't
conveniently bound in vc-dir mode, so I added a binding.  But then I
discovered a couple of bugs in the git back end; and then later
discovered that the same bugs affect vc-git-log-incoming.

First, vc-git-log-incoming and vc-git-log-outgoing can be called with a
nil value for remote-location.  In this case nothing shows up in the
log.  Changing the code to treat nil the same as the empty string makes
the commands show the data I would expect.

Next, these commands move point to the end of the new buffer.  However,
that is weird, because then the buffer is shrunk to fit; for me this
ends up making it look like I have just a single outgoing patch, even
when I have several.  Changing these functions to pass 'async to
vc-git-command fixes this.

Finally, vc-git-log-view-mode only sets truncate-lines in some
situations; I have changed it to also set it for log-outgoing and
log-incoming, which makes the output of these commands more closely
resemble the output of vc-print-log.

Tom