fetch: don't try to update unfetched tracking refs

Since commit f269048 (fetch: opportunistically update tracking refs,
2013-05-11) we update tracking refs opportunistically when fetching
remote branches.  However, if there is a configured non-pattern refspec
that does not match any of the refspecs given on the command line then a
fatal error occurs.

Fix this by setting the "missing_ok" flag when calling get_fetch_map.

Test-added-by: Jeff King <peff@peff.net>
Signed-off-by: John Keeping <john@keeping.me.uk>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
completion: zsh: improve bash script loading

It's better to check in multiple locations, so the user doesn't have to.

And update the documentation.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'fc/zsh-leftover-bits' into next

* fc/zsh-leftover-bits:
  completion: zsh: improve bash script loading
Merge branch 'jk/fetch-always-update-tracking' into next

Hotfix; the main series forgot that what is being fetched in a
one-shot fetch may not be covered by the refspec specified for the
remote.

* jk/fetch-always-update-tracking:
  fetch: don't try to update unfetched tracking refs