Fix minor bugs in vc-git and vc-hg on Windows uncovered by vc-tests

* lisp/vc/vc-hg.el (vc-hg-state-slow): Non-existing files emit a
different message on MS-Windows; support that.
* lisp/vc/vc-git.el (vc-git-checkin): Make sure
'default-directory' is not nil when calling
'make-nearby-temp-file' on MS-Windows.

* test/lisp/vc/vc-tests.el (vc-test--version-diff): Run
'default-directory' through 'file-truename', otherwise the
'vc-test-cvs06-version-diff' test might fail on MS-Windows.

lisp/vc/vc-git.el

@@ -1014,7 +1014,8 @@ It is based on `log-edit-mode', and has Git-specific extensions."
           ;; might not support the non-ASCII characters in the log
           ;; message.  Handle also remote files.
           (if (eq system-type 'windows-nt)
-              (let ((default-directory (file-name-directory file1)))
+              (let ((default-directory (or (file-name-directory file1)
+                                           default-directory)))
                 (make-nearby-temp-file "git-msg")))))
     (when vc-git-patch-string
       (unless (zerop (vc-git-command nil t nil "diff" "--cached" "--quiet"))

lisp/vc/vc-hg.el

@@ -249,7 +249,10 @@ If `ask', you will be prompted for a branch type."
                     (error nil)))))))
     (when (and (eq 0 status)
 	       (> (length out) 0)
-	       (null (string-match ".*: No such file or directory$" out)))
+                         ;; Posix
+	       (null (or (string-match ".*: No such file or directory$" out)
+                         ;; MS-Windows
+                         (string-match ".*: The system cannot find the file specified$" out))))
       (let ((state (aref out 0)))
 	(cond
 	 ((eq state ?=) 'up-to-date)

test/lisp/vc/vc-tests.el

@@ -596,8 +596,9 @@ This checks also `vc-backend' and `vc-responsible-backend'."
     (let ((vc-handled-backends `(,backend))
           (default-directory
            (file-name-as-directory
-            (expand-file-name
-             (make-temp-name "vc-test") temporary-file-directory)))
+            (file-truename
+             (expand-file-name
+              (make-temp-name "vc-test") temporary-file-directory))))
           (process-environment process-environment)
           vc-test--cleanup-hook)
       (when (eq backend 'Bzr)