Rename #Prepare to #ShellCommand

Shell commands are on their way out.  Let's start reclaiming this verb.

plugin/fugitive.vim

@@ -134,7 +134,7 @@ endfunction
 " it will be used as the Git dir.  If it's a buffer number, the Git dir for
 " that buffer will be used.  The default is the current buffer.
 function! FugitivePrepare(...) abort
-  return call('fugitive#Prepare', a:000)
+  return call('fugitive#ShellCommand', a:000)
 endfunction
 
 " FugitiveConfig() get returns an opaque structure that can be passed to other