Fix s:Cd() usage in the Gblame command

Most usages in commit da3b2f32 (Support tab local working directories)
were updated to use this pattern, but Gblame missed the boat. Without
this, Gblame will change the root git tree but won't change back to the
previous location.

autoload/fugitive.vim

@@ -3549,13 +3549,7 @@ function! s:Blame(bang, line1, line2, count, mods, args) abort
     let cmd += ['--', expand('%:p')]
     let basecmd = escape(fugitive#Prepare(cmd), '!#%')
     try
-      let cd = s:Cd()
+      let cdback = s:Cd(s:Tree())
-      let tree = s:Tree()
-      let cdback = s:Cd(tree)
-      if len(tree) && s:cpath(tree) !=# s:cpath(getcwd())
-        let cwd = getcwd()
-        execute cd s:fnameescape(tree)
-      endif
       let error = tempname()
       let temp = error.'.fugitiveblame'
       if &shell =~# 'csh'
@@ -3563,10 +3557,10 @@ function! s:Blame(bang, line1, line2, count, mods, args) abort
       else
         silent! execute '%write !'.basecmd.' > '.temp.' 2> '.error
       endif
-      if exists('l:cwd')
+    finally
-        execute cd s:fnameescape(cwd)
+      execute cdback
-        unlet cwd
+    endtry
-      endif
+    try
       if v:shell_error
         call s:throw(join(readfile(error),"\n"))
       endif
@@ -3639,10 +3633,6 @@ function! s:Blame(bang, line1, line2, count, mods, args) abort
         redraw
         syncbind
       endif
-    finally
-      if exists('l:cwd')
-        execute cd s:fnameescape(cwd)
-      endif
     endtry
     return ''
   catch /^fugitive:/