(vc-next-action): Do not consider directories when

(vc-next-action): Do not consider directories when
checking for state compatibility.
(vc-transfer-file): Use when not if.
(vc-dir-parent-marked-p, vc-dir-children-marked-p): New functions.
(vc-dir-mark-file): Use them.
(vc-deduce-fileset): Also return the backend.
(vc-diff-internal): Take as argument the value returned by
vc-deduce-fileset instead of just the fileset.
(vc-next-action, vc-finish-logentry, vc-version-diff, vc-diff)
(vc-dir-mark-file, vc-print-log, vc-revert, vc-rollback)
(vc-update): Update the vc-deduce-fileset and vc-diff-internal calls.