Make the process-connection-type customizable.

We need process-connection-type to be nil on cygwin
We need process-connection-type to be t for magit to ask for password
So we now use the new variable magit-process-connection-type for this,
defaulting to nil on cygwin, and to t on others platform.

magit.el

@@ -141,6 +141,14 @@ Setting this to nil will make it do nothing, setting it to t will arrange things
   :group 'magit
   :type 'boolean)
 
+(defcustom magit-process-connection-type (not (eq system-type 'cygwin))
+  "Connection type used for the git process.
+
+nil mean pipe, it is usually faster and more efficient, and work on cygwin.
+t mean pty, it enable magit to prompt for passphrase when needed."
+  :group 'magit
+  :type 'boolean)
+
 (defface magit-header
   '((t))
   "Face for generic header lines.
@@ -1226,7 +1234,7 @@ FUNC should leave point at the end of the modified region"
 		"\n")
 	(cond (nowait
 	       (setq magit-process
-		     (let ((process-connection-type nil))
+		     (let ((process-connection-type magit-process-connection-type))
 		       (apply 'start-file-process cmd buf cmd args)))
 	       (set-process-sentinel magit-process 'magit-process-sentinel)
 	       (set-process-filter magit-process 'magit-process-filter)