Add script to verify regressions against a known-good database

When trying to optimize performance, it is really important to
not break the calculations that are being performed. To make this
easier to verify, add a script that takes two project names for
the same repository (intended to be a known-good version and a
new improved version) and verifies that the data computed for
each commit is the same.

Future improvements: Verify the whole database, though verifying
the commits does the important parts. Run this automatically
(will need to wait until proper CI is set up anyway).

Author: Jaakko Kangasharju

tests/check_regression.py

@@ -0,0 +1,35 @@
+import sys
+import itertools
+
+from githammer import Hammer
+
+
+def check_commit(index, commit_old, commit_new, attr):
+    old_attr = getattr(commit_old, attr)
+    new_attr = getattr(commit_new, attr)
+    if old_attr != new_attr:
+        sys.exit('Error in commit {} ({}): Incorrect {} {} (expected {})'.
+                 format(index, commit_old.hexsha, attr, new_attr, old_attr))
+
+
+if len(sys.argv) < 3:
+    sys.exit('Usage: {} <known good project> <new project>'.format(sys.argv[0]))
+
+hammer_old = Hammer(sys.argv[1])
+hammer_new = Hammer(sys.argv[2])
+
+count = 0
+
+for (index, (commit_old, commit_new)) in enumerate(itertools.zip_longest(hammer_old.iter_individual_commits(),
+                                                                         hammer_new.iter_individual_commits())):
+    check_commit(index, commit_old, commit_new, 'hexsha')
+    check_commit(index, commit_old, commit_new, 'author_name')
+    check_commit(index, commit_old, commit_new, 'added_lines')
+    check_commit(index, commit_old, commit_new, 'deleted_lines')
+    check_commit(index, commit_old, commit_new, 'commit_time')
+    check_commit(index, commit_old, commit_new, 'commit_time_utc_offset')
+    check_commit(index, commit_old, commit_new, 'line_counts')
+    check_commit(index, commit_old, commit_new, 'test_counts')
+    count += 1
+
+print('OK, checked {} commits'.format(count))