vc-hg-rename-file does not expand the file name arguments

[Alex Harsanyi <alexharsanyi@gmail.com>]

I believe there is a problem with the implementation of
`vc-hg-rename-file`, in the call:  (vc-hg-command nil 0 new "mv" old),
only the NEW parameter will be expanded using `expand-file-name`,
because `vc-hg-command` treats it as a file. The OLD parameter is not
expanded because it is treated as a flag.  The problem is that
`vc-rename-file` will read a file name from he minibuffer and pass in
a filename like "~/Projects/test-repo/file1.txt" to the function.
This name is passed directly to the "hg" executable which will fail
because the file does not exist (hg does not do file-name expansion
internally).

For example, create the following mercurial repository:

$ mkdir ~/test-repo
$ cd ~/test-repo
$ hg init .
$ echo Hello > ./file1.txt
$ hg add file1.txt
$ hg commit -m "File1 added"

Now, open file1.txt in Emacs, and try to rename it using "M-x
vc-rename-file".  The rename will fail and the *vc* buffer will
contain:

~/test-repo/file1.txt: No such file or directory
abort: no files to copy

I believe the simplest fix is to call `expand-file-name` on the file
names before passing them to the "hg" command.

Based on the comment in the code, I think the same problem exists with
the bzr backend as well.

Cheers,
Alex.