Remove Git dir checks on FileType events

In order for these checks to work, :filetype on must be invoked *after*
Fugitive is loaded.  Otherwise, the FileType event triggers before
FugitiveDetect() is called.

These file types are used almost exclusively inside of Git repositories,
and in the rare case they are not, we're not doing anything particularly
intrusive, so dropping the conditional should have little practical
impact.  An exception is fugitive#MapJumps(), which is designed to be
used exclusively with historical buffers, the status buffer, and
captured :Git output, so let's lock it down to those particular
workflows.

autoload/fugitive.vim

@@ -2273,7 +2273,10 @@ function! s:TempReadPost(file) abort
   if has_key(s:temp_files, s:cpath(a:file))
     let dict = s:temp_files[s:cpath(a:file)]
     setlocal nobuflisted
-    if has_key(dict, 'filetype') && dict.filetype !=# &l:filetype
+    if get(dict, 'filetype', '') ==# 'git'
+      call fugitive#MapJumps()
+    endif
+    if has_key(dict, 'filetype')
       let &l:filetype = dict.filetype
     endif
     setlocal foldmarker=<<<<<<<,>>>>>>>
@@ -6117,6 +6120,9 @@ endfunction
 
 function! s:StatusCfile(...) abort
   let tree = s:Tree()
+  if empty(tree)
+    return ['']
+  endif
   let lead = s:cpath(tree, getcwd()) ? './' : tree . '/'
   let info = s:StageInfo()
   let line = getline('.')
@@ -6146,6 +6152,9 @@ endfunction
 
 function! s:MessageCfile(...) abort
   let tree = s:Tree()
+  if empty(tree)
+    return ''
+  endif
   let lead = s:cpath(tree, getcwd()) ? './' : tree . '/'
   if getline('.') =~# '^.\=\trenamed:.* -> '
     return lead . matchstr(getline('.'),' -> \zs.*')
@@ -6172,6 +6181,9 @@ function! fugitive#MessageCfile() abort
 endfunction
 
 function! s:cfile() abort
+  if empty(FugitiveGitDir())
+    return []
+  endif
   try
     let myhash = s:DirRev(@%)[1]
     if len(myhash)