Use gitlab-friendly URLs when formatting changelogs as HTML

Our old https://bugs.torproject.org/nnnn URLs only work for bugs
numbered before 40000.  Newer gitlab bugs need to have specific
projects mentioned.

This patch assumes that bugs are in tpo/core/tor by default, but
allows us to refer to several other projects by saying
e.g. "chutney#40002" if we want.

scripts/maint/format_changelog.py

@@ -405,10 +405,31 @@ class ChangeLog(object):
         self.dumpEndOfSections()
         self.dumpEndOfChangelog()
 
+# Map from issue prefix to pair of (visible prefix, url prefix)
+ISSUE_PREFIX_MAP = {
+    "" : ( "", "tpo/core/tor" ),
+    "tor#" : ( "", "tpo/core/tor" ),
+    "chutney#" : ( "chutney#", "tpo/core/chutney" ),
+    "torspec#" : ( "torspec#", "tpo/core/torspec" ),
+    "trunnel#" : ( "trunnel#", "tpo/core/trunnel" ),
+    "torsocks#" : ( "torsocks#", "tpo/core/torsocks"),
+}
+
 # Let's turn bugs to html.
-BUG_PAT = re.compile('(bug|ticket|issue|feature)\s+(\d{4,5})', re.I)
+BUG_PAT = re.compile('(bug|ticket|issue|feature)\s+([\w/]+#)?(\d{4,6})', re.I)
 def bug_html(m):
-    return "%s <a href='https://bugs.torproject.org/%s'>%s</a>" % (m.group(1), m.group(2), m.group(2))
+    kind = m.group(1)
+    prefix = m.group(2) or ""
+    bugno = m.group(3)
+    try:
+        disp_prefix, url_prefix = ISSUE_PREFIX_MAP[prefix]
+    except KeyError:
+        print("Can't figure out URL for {}{}".formt(prefix,bugno),
+              file=sys.stderr)
+        return "{} {}{}".format(kind, prefix, bugno)
+
+    return "{} <a href='https://bugs.torproject.org/{}/{}'>{}{}</a>".format(
+        kind, url_prefix, bugno, disp_prefix, bugno)
 
 class HTMLChangeLog(ChangeLog):
     def __init__(self, *args, **kwargs):