Add a minor mode and keybindings to sy-git.

Add a minor mode and keybindings to sy-git.

With this changeset the goodies in sy-git.el are only available in buffers
of files that are under git control.  And new keybindings were added:

  `C-x G a' -- #'sy-git-add-log-entry
  `C-x G b' -- #'sy-git-blame
  `C-x G =' -- #'sy-git-diff
  `C-x G d' -- #'sy-git-diff
  `C-x G e' -- #'sy-git-ediff
  `C-x G l' -- #'sy-git-log

The global binding for #'sy-git-add-log-entry has been removed.

	* sy-git.el (sy-git-mode-map): New.
	(sy-git-mode): New minor mode.
	(sy-git-activate-maybe): New, for use in `find-file-hooks'. Will
	only activate if the visited file is under git control.

Signed-off-by: Steve Youngs <steve@sxemacs.org>