Fire GitGutterStage User event after staging

Closes #698.

README.mkd

@@ -88,6 +88,10 @@ You can jump between hunks with `[c` and `]c`.  You can preview, stage, and undo
 
 You cannot unstage a staged hunk.
 
+After updating the signs, the plugin fires the `GitGutter` User autocommand.
+
+After staging a hunk or part of a hunk, the plugin fires the `GitGutterStage` User autocommand.
+
 
 #### Activation
 

autoload/gitgutter/hunk.vim

@@ -1,4 +1,5 @@
 let s:winid = 0
+let s:nomodeline = (v:version > 703 || (v:version == 703 && has('patch442'))) ? '<nomodeline>' : ''
 
 function! gitgutter#hunk#set_hunks(bufnr, hunks) abort
   call gitgutter#utility#setbufvar(a:bufnr, 'hunks', a:hunks)
@@ -274,6 +275,10 @@ function! s:stage(hunk_diff)
         \ diff)
   if v:shell_error
     call gitgutter#utility#warn('patch does not apply')
+  else
+    if exists('#User#GitGutterStage')
+      execute 'doautocmd' s:nomodeline 'User GitGutterStage'
+    endif
   endif
 
   " Refresh gitgutter's view of buffer.

doc/gitgutter.txt

@@ -177,7 +177,7 @@ Commands for folds:~
 
 
 ===============================================================================
-AUTOCOMMAND                                             *gitgutter-autocommand*
+AUTOCOMMANDS                                           *gitgutter-autocommands*
 
 User GitGutter~
 
@@ -189,6 +189,10 @@ event GitGutter.  You can listen for this event, for example:
 A dictionary `g:gitgutter_hook_context` is made available during its execution,
 which contains an entry `bufnr` that contains the buffer number being updated.
 
+User GitGutterStage~
+
+After staging a hunk or part of a hunk vim-gitgutter fires a |User| |autocmd|
+with the event GitGutterStage.  Staging always happens in the current buffer.
 
 ===============================================================================
 MAPPINGS                                                   *gitgutter-mappings*