Fix git-dir detection on Windows

Closes #176.

plugin/fugitive.vim

@@ -105,8 +105,8 @@ function! fugitive#is_git_dir(path) abort
 endfunction
 
 function! fugitive#extract_git_dir(path) abort
-  if s:shellslash(a:path) =~? '^fugitive://.*//'
-    return matchstr(a:path,'fugitive://\zs.\{-\}\ze//')
+  if s:shellslash(a:path) =~# '^fugitive://.*//'
+    return matchstr(s:shellslash(a:path), '\C^fugitive://\zs.\{-\}\ze//')
   endif
   let root = s:shellslash(simplify(fnamemodify(a:path,':p:s?[\/]$??')))
   let previous = ""