git: quote sendemail section header

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

Also adds a test case.

Fixes #557

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

@@ -0,0 +1,33 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+{
+  imports = [ ../accounts/email-test-accounts.nix ];
+
+  config = {
+    programs.git = {
+      enable = true;
+      userEmail = "hm@example.com";
+      userName = "H. M. Test";
+    };
+
+    nmt.script = ''
+      function assertGitConfig() {
+        local value
+        value=$(${pkgs.git}/bin/git config \
+          --file $TESTED/home-files/.config/git/config \
+          --get $1)
+        if [[ $value != $2 ]]; then
+          fail "Expected option '$1' to have value '$2' but it was '$value'"
+        fi
+      }
+
+      assertFileExists home-files/.config/git/config
+      assertFileContent home-files/.config/git/config ${./git-with-email-expected.conf}
+
+      assertGitConfig "sendemail.hm@example.com.from" "hm@example.com"
+      assertGitConfig "sendemail.hm-account.from" "hm@example.org"
+    '';
+  };
+}