Map cmt to :Git mergetool

References https://github.com/tpope/vim-fugitive/issues/1329

autoload/fugitive.vim

@@ -5946,6 +5946,7 @@ function! fugitive#MapJumps(...) abort
 
     nnoremap <buffer>      cm<Space> :Git merge<Space>
     nnoremap <buffer>         cm<CR> :Git merge<CR>
+    nnoremap <buffer>          cmt   :Git mergetool
     nnoremap <buffer> <silent> cm?   :help fugitive_cm<CR>
 
     nnoremap <buffer>      cz<Space> :Git stash<Space>

doc/fugitive.txt

@@ -111,7 +111,7 @@ that are part of Git repositories).
                         change unless [!] is given.
 
 :Git difftool -y [args] Invoke `git diff [args]`, open each changed file in a
-                        new tab, and invoke `:Gdiffsplit` against the
+                        new tab, and invoke `:Gdiffsplit!` against the
                         appropriate commit.
 
                                                 *:Git-mergetool*