feature: Add title-min-length

* Add a new `title-min-length` rule which mimics the `body-min-length`
  rule in its function. The default min length is set to 5 as per the
  discussion in jorisroovers#138.

* Add a few tests modeled on TitleMaxLength.

Signed-off-by: mr.Shu <mr@shu.io>

gitlint/rules.py

@@ -236,6 +236,20 @@ def validate(self, title, _commit):
             return [RuleViolation(self.id, violation_msg, title)]
 
 
+class TitleMinLength(LineRule):
+    name = "title-min-length"
+    id = "T8"
+    target = CommitMessageTitle
+    options_spec = [IntOption('min-length', 5, "Min line length in characters")]
+
+    def validate(self, title, _commit):
+        min_length = self.options['min-length'].value
+        actual_length = len(title)
+        if 0 < actual_length < min_length:
+            violation_message = "Title is too short ({0}<{1})".format(actual_length, min_length)
+            return [RuleViolation(self.id, violation_message, title, 1)]
+
+
 class BodyMaxLineLength(MaxLineLength):
     name = "body-max-line-length"
     id = "B1"

gitlint/tests/rules/test_title_rules.py

@@ -1,7 +1,7 @@
 # -*- coding: utf-8 -*-
 from gitlint.tests.base import BaseTestCase
 from gitlint.rules import TitleMaxLength, TitleTrailingWhitespace, TitleHardTab, TitleMustNotContainWord, \
-    TitleTrailingPunctuation, TitleLeadingWhitespace, TitleRegexMatches, RuleViolation
+    TitleTrailingPunctuation, TitleLeadingWhitespace, TitleRegexMatches, RuleViolation, TitleMinLength
 
 
 class TitleRuleTests(BaseTestCase):
@@ -152,3 +152,25 @@ def test_regex_matches(self):
         violations = rule.validate(commit.message.title, commit)
         expected_violation = RuleViolation("T7", u"Title does not match regex (^UÅ[0-9]*)", u"US1234: åbc")
         self.assertListEqual(violations, [expected_violation])
+
+    def test_min_line_length(self):
+        rule = TitleMinLength()
+
+        # assert no error
+        violation = rule.validate(u"å" * 72, None)
+        self.assertIsNone(violation)
+
+        # assert error on line length > 72
+        expected_violation = RuleViolation("T8", "Title is too short (4<5)", u"å" * 4, 1)
+        violations = rule.validate(u"å" * 4, None)
+        self.assertListEqual(violations, [expected_violation])
+
+        # set line length to 3, and check no violation on length 4
+        rule = TitleMaxLength({'min-length': 3})
+        violations = rule.validate(u"å" * 4, None)
+        self.assertIsNone(violations)
+
+        # assert raise on 2
+        expected_violation = RuleViolation("T8", "Title is too short (2<3)", u"å" * 2, 1)
+        violations = rule.validate(u"å" * 2, None)
+        self.assertListEqual(violations, [expected_violation])