Include git notes in commit buffers

Issue #912

Add any existing git notes into commit views, between the commit message and
the patch itself.  This is similar to what "git show" does, albeit without any
indent.

Calling "git notes list" serves both to suppress the "Notes:" prefix when a
commit does not have any notes, and to avoid using notes formats in older
versions of git that do not support the feature.

autoload/fugitive.vim

@@ -2031,7 +2031,9 @@ function! fugitive#BufReadCmd(...) abort
         if b:fugitive_display_format
           call s:ReplaceCmd([dir, 'cat-file', b:fugitive_type, rev])
         else
-          call s:ReplaceCmd([dir, 'show', '--no-color', '-m', '--first-parent', '--pretty=format:tree%x20%T%nparent%x20%P%nauthor%x20%an%x20<%ae>%x20%ad%ncommitter%x20%cn%x20<%ce>%x20%cd%nencoding%x20%e%n%n%s%n%n%b', rev])
+          let [note, exec_error] = s:ChompError([dir, 'notes', 'list', rev])
+          let noteformat = (exec_error ? '' : '%nNotes:%n%N')
+          call s:ReplaceCmd([dir, 'show', '--no-color', '-m', '--first-parent', '--pretty=format:tree%x20%T%nparent%x20%P%nauthor%x20%an%x20<%ae>%x20%ad%ncommitter%x20%cn%x20<%ce>%x20%cd%nencoding%x20%e%n%n%s%n%n%b' . noteformat, rev])
           keepjumps call search('^parent ')
           if getline('.') ==# 'parent '
             silent keepjumps delete_