Import gitchanglog 3.0.4

The  project hasn't seen any new development/changes since 2018 and it
seems unlikely we'd be able to get any changes into the upstream. Since
it's isolated into a single file and its task is fairly straighforward,
pull the code into our own repository and maintain it here as needed.

This also makes it easier to make any changes that are specific to our
project.

(cherry picked from commit 63247d8a73)

contrib/README

@@ -27,5 +27,9 @@ be fixed as time permits.
       named at runtime, enabling access to external data sources including
       LDAP, MySQL, Berkeley DB, perl scripts, etc.
 
+    - gitchangelog/
+
+      QA utility to produce changelog and release notes from git log.
+
 Some links to useful software and other resources related to BIND 9 and
 DNS can be found at https://www.isc.org/dns-tools-and-resources.

contrib/gitchangelog/gitchangelog.spdx

@@ -0,0 +1,11 @@
+SPDXVersion: SPDX-2.2
+DataLicense: CC0-1.0
+SPDXID: SPDXRef-DOCUMENT
+DocumentName: gitchangelog
+DocumentNamespace: https://gitlab.isc.org/isc-projects/bind9/-/tree/main/contrib/gitchangelog/
+
+PackageName: gitchangelog
+SPDXID: SPDXRef-Package-gitchangelog
+PackageDownloadLocation: git+https://github.com/vaab/gitchangelog.git@95fe8d5bd171db29922b40d9d3a13dab5af094c0#src/gitchangelog
+PackageOriginator: Person: Valentin Lab <valentin.lab@kalysto.org>
+PackageLicenseDeclared: BSD-3-Clause