When VC detects a conflict, specify which file

When VC detects a conflict, specify which file

* lisp/vc/vc.el (vc-message-unresolved-conflicts): New function.
* lisp/vc/vc-svn.el (vc-svn-find-file-hook):
* lisp/vc/vc-hg.el (vc-hg-find-file-hook):
* lisp/vc/vc-bzr.el (vc-bzr-find-file-hook):
* lisp/vc/vc-git.el (vc-git-find-file-hook): Use above new function
  to display a standard message that specifies the conflicted file.

Before this change, the message VC used for indicating a conflicted
file was just "There are unresolved conflicts in this file" without
naming the file (and this language was duplicated in several places).
After this change, it's "There are unresolved conflicts in file FOO"
(and this language is now centralized in one function in vc.el).

Justification: It's important for the message to name the conflicted
file because the moment when VC realizes a file is conflicted does not
always come interactively.  For example, some people automatically
find a set of Org Mode files on startup, and may keep those .org files
under version control.  If any of the files are conflicted, the user
just sees some messages fly by, and might later check the "*Messages*"
buffer to find out what files were conflicted.  I'm not saying this
happened to me or anything; it's a purely hypothetical example.