Teach a python script about git work-trees (#1794)

With git worktrees, `.git` is actually a text file that contains a kind
of user-space symlink to the parent.

Fixes: #1793

emsdk.py

diff --git a/emsdk.py b/emsdk.py
index b41a0c4..9331135 100644
--- a/emsdk.py
+++ b/emsdk.py
@@ -770,7 +770,7 @@
 
 def git_clone(url, dstpath, branch, remote_name='origin'):
   debug_print(f'git_clone(url={url}, dstpath={dstpath})')
-  if os.path.isdir(os.path.join(dstpath, '.git')):
+  if os.path.exists(os.path.join(dstpath, '.git')):
     remotes = get_git_remotes(dstpath)
     if remote_name in remotes:
       debug_print(f'Repository {url} with remote "{remote_name}" already cloned to directory {dstpath}, skipping.')