Add test for commit message format

Added test that checks if commits follow the 50/72 git
commit rule and if they are signed (git commit -s).
Fixes: #9, #14.

Signed-off-by: Laura Loghin <lauralg@amazon.com>

integration_tests/test_commit_format.py

@@ -0,0 +1,62 @@
+# Copyright 2020 Amazon.com, Inc. or its affiliates. All Rights Reserved.
+# SPDX-License-Identifier: Apache-2.0
+"""Test the commit message format."""
+
+import os
+
+from utils import get_cmd_output
+
+COMMIT_TITLE_MAX_LEN = 50
+COMMIT_BODY_LINE_MAX_LEN = 72
+BASE_BRANCH = os.environ['BUILDKITE_PULL_REQUEST_BASE_BRANCH']
+
+
+def test_commit_format():
+    """
+    Checks commit message format for the current PR's commits.
+
+    Checks if commit messages follow the 50/72 git commit rule
+    [https://www.midori-global.com/blog/2018/04/02/git-50-72-rule]
+    and if commits are signed.
+    """
+    # Get hashes of PR's commits in their abbreviated form for
+    # a prettier printing.
+    shas_cmd = "git log --no-merges --pretty=%h --no-decorate " \
+               "{}..HEAD".format(BASE_BRANCH)
+    shas = get_cmd_output(shas_cmd)
+
+    for sha in shas.split():
+        message_cmd = "git show --pretty=format:%B -s " + sha
+        message = get_cmd_output(message_cmd)
+        message_lines = message.split("\n")
+        assert len(message_lines) >= 3,\
+            "The commit should contain at least 3 lines: title, blank " \
+            "line and a sign-off one. Please check: " \
+            "https://www.midori-global.com/blog/2018/04/02/git-50-72-rule."
+        title = message_lines[0]
+        assert message_lines[1] == "", "Commit title is divided into " \
+                                       "multiple lines. Please keep it " \
+                                       "one line long and make sure you " \
+                                       "add a blank line between title " \
+                                       "and description."
+        assert len(title) <= COMMIT_TITLE_MAX_LEN,\
+            "For commit '{}', title exceeds {} chars. " \
+            "Please keep it shorter.".format(sha, COMMIT_TITLE_MAX_LEN)
+
+        found_signed_off = False
+
+        for line in message_lines[2:]:
+            if line.startswith("Signed-off-by: "):
+                found_signed_off = True
+                # If we found `Signed-off-by` line, then it means
+                # the commit message ended and we don't want to check
+                # line lengths anymore for the current commit.
+                break
+            assert len(line) <= COMMIT_BODY_LINE_MAX_LEN,\
+                "For commit '{}', message line '{}' exceeds {} chars. " \
+                "Please keep it shorter or split it in " \
+                "multiple lines.".format(sha, line,
+                                         COMMIT_BODY_LINE_MAX_LEN)
+        assert found_signed_off, "Commit '{}' is not signed. " \
+                                 "Please run 'git commit -s --amend' " \
+                                 "on it.".format(sha)