git_config: hoist Windows ssh check earlier

The ssh master logic has never worked under Windows which is why this
code always returned False when running there (including cygwin).  But
the OS check was still done while holding the threading lock.  While
it might be a little slower than necessary, it still worked.

The switch from the threading module to the multiprocessing module
changed global behavior subtly under Windows and broke things: the
globals previously would stay valid, but now they get cleared.  So
the lock is reset to None in children workers.

We could tweak the logic to pass the lock through, but there isn't
much point when the rest of the code is still disabled in Windows.
So perform the platform check before we grab the lock.  This fixes
the crash, and probably speeds things up a few nanoseconds.

This shouldn't be a problem on Linux systems as the platform fork
will duplicate the existing process memory (including globals).

Bug: https://crbug.com/gerrit/14480
Change-Id: I1d1da82c6d7bd6b8cdc1f03f640a520ecd047063
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/305149
Reviewed-by: Raman Tenneti <rtenneti@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

git_config.py

@@ -459,6 +459,11 @@ def init_ssh():
 def _open_ssh(host, port=None):
   global _ssh_master
 
+  # Bail before grabbing the lock if we already know that we aren't going to
+  # try creating new masters below.
+  if sys.platform in ('win32', 'cygwin'):
+    return False
+
   # Acquire the lock.  This is needed to prevent opening multiple masters for
   # the same host when we're running "repo sync -jN" (for N > 1) _and_ the
   # manifest <remote fetch="ssh://xyz"> specifies a different host from the
@@ -476,11 +481,8 @@ def _open_ssh(host, port=None):
     if key in _master_keys:
       return True
 
-    if (not _ssh_master
-            or 'GIT_SSH' in os.environ
-            or sys.platform in ('win32', 'cygwin')):
-      # failed earlier, or cygwin ssh can't do this
-      #
+    if not _ssh_master or 'GIT_SSH' in os.environ:
+      # Failed earlier, so don't retry.
       return False
 
     # We will make two calls to ssh; this is the common part of both calls.