(vc-update-change-log): Check that ChangeLog is writable

(vc-update-change-log): Check that ChangeLog is writable
before starting the expensive rcs2log process.
Use call-process instead of shell-command to invoke rcs2log;
this avoids undesired shell escapes and is more robust about errors.
Put mark at point-min, so that the new insertion is in the region.
(vc-checkin-hook): Fix `runs-hooks' typo.
(vc-checkout-writeable-buffer-hook): New var.
(vc-next-action): Fix bug: initial checkin was botched when C-x v v
was applied to a new file while vc-initial-comment was non-nil.
(vc-register): Don't barf when registering a new, empty buffer.
(vc-directory): The `No files are currently registered'
message was wrongly worded, because sometimes the message talks
about locked files, not registered files.
(vc-file-tree-walk): Change (apply 'funcall ...) to (apply
...), since the 'funcall is redundant.
When traversing a directory tree, message
"Traversing directory XXX" so that the user can see what progress is
being made.  Traversal can take a long time.  Omit first argument,
since it is always the current directory.  All callers changed.
(vc-file-tree-walk-internal): New function.
(vc-do-command, vc-diff, vc-version-diff, vc-backend-diff):
Remove redundant calls to `format'.
(vc-diff): Remove unused variable `old'.
(vc-version-diff): When recursively generating a difference
listing, don't append the latest output unless diff was actually run;
otherwise, you'll get the output from the previous file by mistake.