Introduce helper to normalize to backslashes on win32

FugitiveVimPath() is intended to potentially convert between UNIX and
Windows paths in mixed environments.  Let's separate uses that require
that from those that simply normalize slashes.

plugin/fugitive.vim

@@ -58,7 +58,7 @@ function! FugitiveReal(...) abort
   if type(file) ==# type({})
     let dir = FugitiveGitDir(file)
     let tree = s:Tree(dir)
-    return FugitiveVimPath(empty(tree) ? dir : tree)
+    return s:VimSlash(empty(tree) ? dir : tree)
   elseif file =~# '^\a\a\+:' || a:0 > 1
     return call('fugitive#Real', [file] + a:000[1:-1])
   elseif file =~# '^/\|^\a:\|^$'
@@ -490,28 +490,42 @@ function! FugitiveDetect(...) abort
   return ''
 endfunction
 
-function! FugitiveVimPath(path) abort
-  if exists('+shellslash') && !&shellslash
-    return tr(a:path, '/', '\')
-  else
-    return a:path
-  endif
-endfunction
-
 function! FugitiveGitPath(path) abort
   return s:Slash(a:path)
 endfunction
 
 if exists('+shellslash')
+
   let s:dir_commit_file = '\c^fugitive://\%(/\a\@=\)\=\(.\{-\}\)//\%(\(\x\{40,\}\|[0-3]\)\(/.*\)\=\)\=$'
+
   function! s:Slash(path) abort
     return tr(a:path, '\', '/')
   endfunction
+
+  function! s:VimSlash(path) abort
+    return tr(a:path, '\/', &shellslash ? '//' : '\\')
+  endfunction
+
+  function FugitiveVimPath(path) abort
+    return tr(a:path, '\/', &shellslash ? '//' : '\\')
+  endfunction
+
 else
+
   let s:dir_commit_file = '\c^fugitive://\(.\{-\}\)//\%(\(\x\{40,\}\|[0-3]\)\(/.*\)\=\)\=$'
+
   function! s:Slash(path) abort
     return a:path
   endfunction
+
+  function! s:VimSlash(path) abort
+    return a:path
+  endfunction
+
+  function! FugitiveVimPath(path) abort
+    return a:path
+  endfunction
+
 endif
 
 function! s:ProjectionistDetect() abort