Improvements to vc-bzr conffile handling and pull/merge support.

Improvements to vc-bzr conffile handling and pull/merge support.

* vc/vc-bzr.el (vc-bzr--branch-conf): Function deleted.
(vc-bzr-branch-conf): New function, similar to vc-bzr--branch-conf
but returning an alist.  Ignore comments in bzr conffile.
(vc-bzr-pull, vc-bzr-merge-branch): Use vc-bzr-branch-conf.
(vc-bzr-error-regex-alist): New var.
(vc-bzr-merge-branch): Use it to highlight the pull/merge buffer.

* progmodes/compile.el (compilation--flush-directory-cache):
Handle the case where cdr of compilation--flush-directory-cache
points to no buffer, which can occur if we previously switched to
compilation-mode in a pregenerated buffer.

* vc/vc-dispatcher.el (vc-do-async-command): Bind
inhibit-read-only to t.