SignifyDiff: cd into dir of file

All VCS assume slightly different things where things have to be if you're in
this directory using that command etc.

Changing to the directory of the current file avoids many pitfalls.

autoload/sy/repo.vim

@@ -260,8 +260,14 @@ function! sy#repo#diffmode() abort
   let ft = &filetype
   tabedit %
   diffthis
+  let [cwd, chdir] = sy#util#chdir()
+  try
+    execute chdir fnameescape(b:sy_info.dir)
     leftabove vnew
     silent put =system(cmd)
+  finally
+    execute chdir fnameescape(cwd)
+  endtry
   silent 1delete
   diffthis
   set buftype=nofile bufhidden=wipe nomodified