More powerful validation for changelog

I've had some trouble with the changelog when rebasing or merging,
so I thought I'd write some more checks.  Now the expected entry
must appear first, all version numbers must be unique and in order,
and no version numbers may be skipped.

See https://xkcd.com/208/

scripts/check-changelog.py

@@ -20,6 +20,7 @@
 from __future__ import division, print_function, absolute_import
 
 import os
+import re
 import sys
 from datetime import datetime, timedelta
 
@@ -34,18 +35,56 @@
         print('No source changes found')
         sys.exit(0)
 
+    found_problem = False
+
     now = datetime.utcnow()
     hour = timedelta(hours=1)
     acceptable_lines = sorted(set(
         '{} - {}'.format(tools.__version__, d.strftime('%Y-%m-%d'))
         for d in (now, now + hour, now - hour)
     ))
 
-    for line in tools.changelog().split('\n'):
-        if line.strip() in acceptable_lines:
+    pattern = r'\n\d+\.\d+\.\d+ - \d{4}-\d{2}-\d{2}\n'
+    all_versions = list(map(str.strip, re.findall(pattern, tools.changelog())))
+
+    for line in acceptable_lines:
+        if line == all_versions[0]:
+            break
+        elif line in all_versions:
+            print('Changelog entry %r is not the first entry!  Check for '
+                  'merge errors.' % line)
+            found_problem = True
             break
     else:
         print('No line with version and current date (%s) in the changelog. '
               'Remember this will be released as soon as you merge to master!'
               % ' or '.join(repr(line) for line in acceptable_lines))
-        sys.exit(1)
+        found_problem = True
+
+    if all_versions != sorted(all_versions, reverse=True):
+        print('You edited old release notes and changed the ordering!')
+        found_problem = True
+
+    v_nums = [v.split(' - ')[0] for v in all_versions]
+    duplicates = sorted(set(v for v in v_nums if v_nums.count(v) > 1))
+    if duplicates:
+        plural = 's have' if len(duplicates) > 1 else ' has'
+        print('The following version%s multiple entries in the '
+              'changelog: %s' % (plural, ', '.join(map(repr, duplicates))))
+        found_problem = True
+
+    skipped = []
+    for this, last in zip(v_nums, v_nums[1:]):
+        maj, minor, patch = map(int, last.split('.'))
+        if maj <= 1:
+            break
+        next_patch, next_minor, next_major = ['%s.%s.%s' % v for v in [
+            (maj, minor, patch + 1), (maj, minor + 1, 0), (maj + 1, 0, 0)]]
+        if this not in (next_patch, next_minor, next_major):
+            skipped.append('Version %r found after %r, expected %r or %r'
+                           % (this, last, next_patch, next_minor))
+            found_problem = True
+    if skipped:
+        print('\n'.join(skipped))
+
+    sys.exit(int(found_problem))