Remove unmerged files on :Gstatus U

doc/fugitive.txt

@@ -60,6 +60,7 @@ that are part of Git repositories).
                         U     |:Git| checkout
                         U     |:Git| checkout HEAD (staged files)
                         U     |:Git| clean (untracked files)
+                        U     |:Git| rm (unmerged files)
 
                                                 *fugitive-:Gcommit*
 :Gcommit [args]         A wrapper around git-commit.  If there is nothing

plugin/fugitive.vim

@@ -864,6 +864,8 @@ function! s:StageUndo() abort
   if !empty(hash)
     if section ==# 'untracked'
       call repo.git_chomp_in_tree('clean', '--', filename)
+    elseif section ==# 'unmerged'
+      call repo.git_chomp_in_tree('rm', '--', filename)
     elseif section ==# 'unstaged'
       call repo.git_chomp_in_tree('checkout', '--', filename)
     else