* lisp/vc/vc-hooks.el (vc-mode-line-state): New function with code moved from
'vc-default-mode-line-string'.
(vc-default-mode-line-string): Use 'vc-mode-line-state'.
* lisp/vc/vc-git.el (vc-git-mode-line-string): Use
'vc-mode-line-state' instead of hacking the string returned from
'vc-default-mode-line-string'.
* lisp/vc/vc-hg.el (vc-hg-mode-line-string): Use
'vc-mode-line-state' instead of duplicating code from
'vc-default-mode-line-string'.