Improve GitGutter User autocommand: context/unsilent

This provides `g:gitgutter_hook_context` during the hook's execution and
removes the `:silent`, but uses `exists()` instead.

The bufnr might be necessary to know in the User autocommand, e.g. to
clear some cache.
Not using`:silent` is good practice in general to not hide (wanted)
output and errors etc.

doc/gitgutter.txt

@@ -159,6 +159,8 @@ event GitGutter.  You can listen for this event, for example:
 >
   autocmd User GitGutter call updateMyStatusLine()
 <
+A dictionary `g:gitgutter_hook_context` is made available during its execution,
+which contains an entry `bufnr` that contains the buffer number being updated.
 
 
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