Cleanup for vc-ignore.

Cleanup for vc-ignore.

* vc/vc.el (vc-ignore): Rewrite.
(vc-default-ignore-completion-table):
(vc--read-lines):
(vc--add-line, vc--remove-regexp): New functions.

* vc/vc-svn.el (vc-svn-ignore): Doc fix.
(vc-svn-ignore-completion-table): New function.

* vc/vc-hg.el (vc-hg-ignore): Rewrite.
(vc-hg-ignore-completion-table):
(vc-hg-find-ignore-file): New functions.

* vc/vc-git.el (vc-git-ignore): Rewrite.
(vc-git-ignore-completion-table):
(vc-git-find-ignore-file): New functions.

* vc/vc-dir.el (vc-dir-menu-map): Add menu for vc-dir-ignore.

* vc/vc-bzr.el (vc-bzr-ignore): Rewrite.
(vc-bzr-ignore-completion-table):
(vc-bzr-find-ignore-file): New functions.