credential-cache: handle ECONNREFUSED gracefully

In 245670c (credential-cache: check for windows specific errors, 2021-09-14)
we concluded that on Windows we would always encounter ENETDOWN where we
would expect ECONNREFUSED on POSIX systems, when connecting to unix sockets.
As reported in [1], we do sometimes encounter ECONNREFUSED on Windows aswell,
though. we should handle these cases like we do on non-windows systems.

[1] #4762 (comment)

This fixes #5314

Helped-by: M Hickford <mirth.hickford@gmail.com>
Signed-off-by: Matthias Aßhauer <mha1993@live.de>

builtin/credential-cache.c

@@ -23,7 +23,7 @@ static int connection_closed(int error)
 
 static int connection_fatally_broken(int error)
 {
-	return (error != ENOENT) && (error != ENETDOWN);
+	return (error != ENOENT) && (error != ENETDOWN) && (error != ECONNREFUSED);
 }
 
 #else