git: quote sendemail section header

This will allow, e.g., the character `@` in the email identity.

Also adds a test case.

Fixes #557

modules/programs/git.nix

@@ -151,7 +151,7 @@ in
             hasSmtp = name: account: account.smtp != null;
 
             genIdentity = name: account: with account;
-              nameValuePair "sendemail.${name}" ({
+              nameValuePair "sendemail \"${name}\"" ({
                 smtpEncryption = if smtp.tls.enable then "tls" else "";
                 smtpServer = smtp.host;
                 smtpUser = userName;