Sort changelog & relnotes entries by issue number

To reduce the friction when handling the release notes, it is preferable
to have the sections sorted by issue number, rather than merge order.
Fallback to commit subject line if unavailable (e.g. for changelog
entries).

(cherry picked from commit 759948fffe)

contrib/gitchangelog/gitchangelog.py

@@ -1802,6 +1802,13 @@ def versions_data_iter(
             ):
                 continue
 
+            body = body_process(commit.body)
+
+            ## Extract gitlab issue number
+            issue = None
+            if match := re.search(r".*:gl:`#([0-9]+)`", body):
+                issue = int(match.group(1))
+
             matched_section = first_matching(section_regexps, commit.subject)
 
             ## Finally storing the commit in the matching section
@@ -1811,11 +1818,21 @@ def versions_data_iter(
                     "author": commit.author_name,
                     "authors": commit.author_names,
                     "subject": subject_process(commit.subject),
-                    "body": body_process(commit.body),
+                    "body": body,
                     "commit": commit,
+                    "issue": issue,
                 }
             )
 
+        ## Sort sections by issue number or title
+        for section_key in sections.keys():
+            sections[section_key].sort(
+                key=lambda c: (
+                    c["issue"] if c["issue"] is not None else sys.maxsize,
+                    c["subject"],
+                )
+            )
+
         ## Flush current version
         current_version["sections"] = [
             {"label": k, "commits": sections[k]} for k in section_order if k in sections
@@ -1830,7 +1847,7 @@ def changelog(
     unreleased_version_label="unreleased",
     include_commit_sha=False,
     warn=warn,  ## Mostly used for test
-    **kwargs
+    **kwargs,
 ):
     """Returns a string containing the changelog of given repository