VC: Use symbolic names for the working revision

* lisp/vc/vc-hg.el (vc-hg-diff): When OLDVERS is ".", behave the
same as if OLDVERS was the actual working revision.
(vc-hg-previous-revision): Return ".~1" for input ".", ".~3" for
input ".^^" et cetera.
Also, when passed non-symbolic revisions, use "REV~1" instead of
"REV^" for MS-Windows compatibility.
(vc-hg-working-revision-symbol):
* lisp/vc/vc-git.el (vc-git-working-revision-symbol):
* lisp/vc/vc-hooks.el (vc-symbolic-working-revision): New
functions.
(vc-default-mode-line-string):
* lisp/vc/ediff-vers.el (ediff-vc-merge-internal):
* test/lisp/vc/vc-tests/vc-tests.el (vc-test--checkin-patch):
* lisp/vc/vc.el (vc-diff-build-argument-list-internal)
(vc-diff-outgoing, vc-revision-other-window, vc-default-revert):
Call vc-symbolic-working-revision.
(vc-buffer-revision): Specify that this should always be a
revision number/hash, not a symbolic name.
* lisp/vc/vc-git.el (vc-git-previous-revision): Return "HEAD~1"
for input "HEAD", "HEAD~3" for input "HEAD^^" et cetera.

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