Re: vc-update for bzr etc.

[Chong Yidong]

I've checked in the support for using vc-merge and vc-update with
DVCSes.  To summarize, this relies on two new state-changing VC
functions, vc-BACKEND-merge-branch (vc-BACKEND-merge was already taken,
for per-file merges) and vc-BACKEND-pull.

If the new functions are available, vc-merge/vc-update uses them; a
prefix argument prompts the user for the exact command to run, to allow
specifying flags or non-default source locations.  If the new functions
are unavailable, the old behavior (per file merges and updates) is
attempted.

vc-pull is an alias for vc-update.  (Should we make it the reverse?)

vc-bzr-pull and vc-bzr-merge-branch run asynchronously, outputting to a
display buffer.  Linking the filenames is desired but not implemented.
Maybe we should use compilation-mode for this, so that next-error can be
used to visit file conflicts?

Help refining this would be welcome.  And with the corresponding
features for other DVCS's.

Also, do I need to do anything special to update VC-dir buffers after
merging or pulling?