git: set SSL if useStartTls is false

The git-send-email [0] script uses StartTLS if `smtpEncryption` is set
to `tls`, which can break services that don't support StartTLS.

[0]: bd42bbe1a4/git-send-email.perl (L1533)

PR #1395

tests/modules/programs/git/git-with-email-expected.conf

@@ -6,7 +6,7 @@
 
 [sendemail "hm@example.com"]
 	from = "hm@example.com"
-	smtpEncryption = "tls"
+	smtpEncryption = "ssl"
 	smtpServer = "smtp.example.com"
 	smtpUser = "home.manager"
 

tests/modules/programs/git/git-with-email.nix

@@ -13,6 +13,8 @@ with lib;
       userName = "H. M. Test";
     };
 
+    home.stateVersion = "20.09";
+
     nmt.script = ''
       function assertGitConfig() {
         local value