Improve directory changing

On Windows, Vim and Nvim can't work with mklink'ed paths. Issue an error message
and bail out before starting the actual job when the directory can't be changed.

Fixes https://github.com/mhinz/vim-signify/issues/279

autoload/sy/repo.vim

@@ -73,13 +73,23 @@ function! sy#repo#get_diff_start(vcs) abort
     endif
 
     let [cmd, options] = s:initialize_job(a:vcs)
+    let [cwd, chdir] = sy#util#chdir()
 
     call sy#verbose(printf('CMD: %s | CWD: %s', string(cmd), b:sy.info.dir), a:vcs)
+
+    try
+      execute chdir fnameescape(b:sy.info.dir)
+    catch
+      echohl ErrorMsg
+      echomsg 'signify: Switching Changing failed: '. b:sy.info.dir
+      echohl NONE
+      return
+    endtry
     let b:sy_job_id_{a:vcs} = jobstart(cmd, extend(options, {
-          \ 'cwd':       b:sy.info.dir,
           \ 'on_stdout': function('s:callback_nvim_stdout'),
           \ 'on_exit':   function('s:callback_nvim_exit'),
           \ }))
+    execute chdir fnameescape(cwd)
 
   " Newer Vim
   elseif has('patch-7.4.1967')
@@ -90,19 +100,23 @@ function! sy#repo#get_diff_start(vcs) abort
     let [cmd, options] = s:initialize_job(a:vcs)
     let [cwd, chdir] = sy#util#chdir()
 
+    call sy#verbose(printf('CMD: %s | CWD: %s', string(cmd), getcwd()), a:vcs)
+
     try
       execute chdir fnameescape(b:sy.info.dir)
-      call sy#verbose(printf('CMD: %s | CWD: %s', string(cmd), getcwd()), a:vcs)
-      let opts = {
-            \ 'in_io':    'null',
-            \ 'out_cb':   function('s:callback_vim_stdout', options),
-            \ 'close_cb': function('s:callback_vim_close', options),
-            \ }
-      let b:sy_job_id_{a:vcs} = job_start(cmd, opts)
     catch
-    finally
+      echohl ErrorMsg
-      execute chdir fnameescape(cwd)
+      echomsg 'signify: Changing directory failed: '. b:sy.info.dir
+      echohl NONE
+      return
     endtry
+    let opts = {
+          \ 'in_io':    'null',
+          \ 'out_cb':   function('s:callback_vim_stdout', options),
+          \ 'close_cb': function('s:callback_vim_close', options),
+          \ }
+    let b:sy_job_id_{a:vcs} = job_start(cmd, opts)
+    execute chdir fnameescape(cwd)
 
   " Older Vim
   else