Add GitGutterDiffOrig command

Closes #827.

README.mkd

@@ -238,6 +238,11 @@ Finally, you can force vim-gitgutter to update its signs across all visible buff
 See the customisation section below for how to change the defaults.
 
 
+### Vimdiff
+
+Use the `GitGutterDiffOrig` command to open a vimdiff view of the current buffer, respecting `g:gitgutter_diff_relative_to` and `:gitgutter_diff_base`.
+
+
 ### Folding
 
 Use the `GitGutterFold` command to fold all unchanged lines, leaving just the hunks visible.  Use `zr` to unfold 3 lines of context above and below a hunk.

autoload/gitgutter.vim

@@ -233,3 +233,29 @@ function! gitgutter#quickfix(current_file)
     call setloclist(0, locations)
   endif
 endfunction
+
+
+function! gitgutter#difforig()
+  let bufnr = bufnr('')
+  let path = gitgutter#utility#repo_path(bufnr, 1)
+
+  vertical new
+  set buftype=nofile
+
+  if g:gitgutter_diff_relative_to ==# 'index'
+    let index_name = gitgutter#utility#get_diff_base(bufnr).':'.path
+    let cmd = gitgutter#utility#cd_cmd(bufnr,
+          \ g:gitgutter_git_executable.' '.g:gitgutter_git_args.' --no-pager show '.index_name
+          \ )
+    " NOTE: this uses &shell to execute cmd.  Perhaps we should use instead
+    " gitgutter#utility's use_known_shell() / restore_shell() functions.
+    silent! execute "read ++edit !" cmd
+  else
+    silent! execute "read ++edit" path
+  endif
+
+  0d_
+  diffthis
+  wincmd p
+  diffthis
+endfunction

doc/gitgutter.txt

@@ -221,6 +221,12 @@ Commands for folds:~
 :GitGutterFold          Fold all unchanged lines.  Execute again to undo.
 
 
+Other commands:~
+
+                                                 *gitgutter-:GitGutterDiffOrig*
+:GitGutterDiffOrig      Similar to |:DiffOrig| but shows gitgutter's diff.
+
+
 ===============================================================================
 AUTOCOMMANDS                                           *gitgutter-autocommands*
 

plugin/gitgutter.vim

@@ -134,6 +134,8 @@ command! -bar GitGutterBufferToggle  call gitgutter#buffer_toggle()
 command! -bar GitGutterQuickFix call gitgutter#quickfix(0)
 command! -bar GitGutterQuickFixCurrentFile call gitgutter#quickfix(1)
 
+command! -bar GitGutterDiffOrig call gitgutter#difforig()
+
 " }}}
 
 " Line highlights {{{