scripts: add gitlint to check for validity of commit messages

Install gitlint using pip:

 # pip install gitlint
 # gitlint install-hook

This will install the pre-commit hook.

Policies are define in .gitlint. Custom rules are available under
scripts/gitlint.

This script will also run in CI, so avoid CI errors by using the hook
above.

Change-Id: I62750a1fd9369341db29c413a6c4a1677bb0db8a
Signed-off-by: Anas Nashif <anas.nashif@intel.com>

.gitlint

@@ -0,0 +1,52 @@
+# All these sections are optional, edit this file as you like.
+[general]
+ignore=title-trailing-punctuation, T3
+# verbosity should be a value between 1 and 3, the commandline -v flags take precedence over this
+verbosity = 2
+# By default gitlint will ignore merge commits. Set to 'false' to disable.
+ignore-merge-commits=true
+# Enable debug mode (prints more output). Disabled by default
+debug = false
+
+# Set the extra-path where gitlint will search for user defined rules
+# See http://jorisroovers.github.io/gitlint/user_defined_rules for details
+extra-path=scripts/gitlint
+
+
+[body-max-line-count]
+max-line-count=200
+
+[title-max-length]
+line-length=72
+
+[title-must-not-contain-word]
+# Comma-separated list of words that should not occur in the title. Matching is case
+# insensitive. It's fine if the keyword occurs as part of a larger word (so "WIPING"
+# will not cause a violation, but "WIP: my title" will.
+words=wip,title
+
+[title-match-regex]
+# python like regex (https://docs.python.org/2/library/re.html) that the
+# commit-msg title must be matched to.
+# Note that the regex can contradict with other rules if not used correctly
+# (e.g. title-must-not-contain-word).
+#regex=^US[0-9]*
+
+[B1]
+# B1 = body-max-line-length
+line-length=72
+
+[body-min-length]
+min-length=3
+
+[body-is-missing]
+# Whether to ignore this rule on merge commits (which typically only have a title)
+# default = True
+ignore-merge-commits=false
+
+[body-changed-file-mention]
+# List of files that need to be explicitly mentioned in the body when they are changed
+# This is useful for when developers often erroneously edit certain files or git submodules.
+# By specifying this rule, developers can only change the file when they explicitly reference
+# it in the commit message.
+#files=gitlint/rules.py,README.md

scripts/gitlint/zephyr_commit_rules.py

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+from gitlint.rules import CommitRule, RuleViolation
+from gitlint.options import IntOption
+
+"""
+The classes below are examples of user-defined CommitRules. Commit rules are gitlint rules that
+act on the entire commit at once. Once the rules are discovered, gitlint will automatically take care of applying them
+to the entire commit. This happens exactly once per commit.
+
+A CommitRule contrasts with a LineRule (see examples/my_line_rules.py) in that a commit rule is only applied once on
+an entire commit. This allows commit rules to implement more complex checks that span multiple lines and/or checks
+that should only be done once per gitlint run.
+
+While every LineRule can be implemented as a CommitRule, it's usually easier and more concise to go with a LineRule if
+that fits your needs.
+"""
+
+
+class BodyMaxLineCount(CommitRule):
+    # A rule MUST have a human friendly name
+    name = "body-max-line-count"
+
+    # A rule MUST have an *unique* id, we recommend starting with UC (for User-defined Commit-rule).
+    id = "UC1"
+
+    # A rule MAY have an option_spec if its behavior should be configurable.
+    options_spec = [IntOption('max-line-count', 3, "Maximum body line count")]
+
+    def validate(self, commit):
+        line_count = len(commit.message.body)
+        max_line_count = self.options['max-line-count'].value
+        if line_count > max_line_count:
+            message = "Body contains too many lines ({0} > {1})".format(line_count, max_line_count)
+            return [RuleViolation(self.id, message, line_nr=1)]
+
+
+class SignedOffBy(CommitRule):
+    """ This rule will enforce that each commit contains a "Signed-Off-By" line.
+    We keep things simple here and just check whether the commit body contains a line that starts with "Signed-Off-By".
+    """
+
+    # A rule MUST have a human friendly name
+    name = "body-requires-signed-off-by"
+
+    # A rule MUST have an *unique* id, we recommend starting with UC (for User-defined Commit-rule).
+    id = "UC2"
+
+    def validate(self, commit):
+        for line in commit.message.body:
+            if line.lower().startswith("signed-off-by"):
+                return
+
+        return [RuleViolation(self.id, "Body does not contain a 'Signed-Off-By' line", line_nr=1)]