Allow tags prefixed with v in default regex

The version scheme v1.0.0 is not an uncommon way to tag versions
and used to be part of the semver spec. This patch allows the
optional v prefix in the default version scheme.

ref: https://github.com/semver/semver/blob/master/semver.md#is-v123-a-semantic-version

Change-Id: I4a99bda8b788ee5fd2a8aca3463eb548d07f1313
Signed-off-by: Thanh Ha <zxiiro@linux.com>

releasenotes/notes/tag-format-bd5018a813c804fd.yaml

@@ -0,0 +1,4 @@
+---
+features:
+  - |
+    Allow optional 'v' prefix in the default version tag regex.

reno/config.py

@@ -66,7 +66,7 @@ _OPTIONS = [
 
     Opt('release_tag_re',
         textwrap.dedent('''\
-        ((?:[\d.ab]|rc)+)  # digits, a, b, and rc cover regular and
+        ((?:v?[\d.ab]|rc)+)  # digits, a, b, and rc cover regular and
                            # pre-releases
         '''),
         textwrap.dedent("""\
@@ -77,7 +77,7 @@ _OPTIONS = [
 
     Opt('pre_release_tag_re',
         textwrap.dedent('''\
-        (?P<pre_release>\.\d+(?:[ab]|rc)+\d*)$
+        (?P<pre_release>\.v?\d+(?:[ab]|rc)+\d*)$
         '''),
         textwrap.dedent("""\
         The regex pattern used to check if a valid release version tag

reno/tests/test_scanner.py

@@ -273,6 +273,20 @@ class BasicTest(Base):
             results,
         )
 
+    def test_tag_with_v_prefix(self):
+        filename = self._add_notes_file()
+        self.repo.git('tag', '-s', '-m', 'tag with v prefix', 'v1.0.0')
+        self.scanner = scanner.Scanner(self.c)
+        raw_results = self.scanner.get_notes_by_version()
+        results = {
+            k: [f for (f, n) in v]
+            for (k, v) in raw_results.items()
+        }
+        self.assertEqual(
+            {'v1.0.0': [filename]},
+            results,
+        )
+
     def test_note_commit_after_tag(self):
         self._make_python_package()
         self.repo.git('tag', '-s', '-m', 'first tag', '1.0.0')
@@ -876,6 +890,23 @@ class PreReleaseTest(Base):
             results,
         )
 
+    def test_tag_with_v_prefix(self):
+        self._make_python_package()
+        self.repo.git('tag', '-s', '-m', 'first tag', 'v1.0.0.0a1')
+        f1 = self._add_notes_file('slug1')
+        self.repo.git('tag', '-s', '-m', 'first tag', 'v1.0.0.0a2')
+        self.scanner = scanner.Scanner(self.c)
+        raw_results = self.scanner.get_notes_by_version()
+        results = {
+            k: [f for (f, n) in v]
+            for (k, v) in raw_results.items()
+        }
+        self.assertEqual(
+            {'v1.0.0.0a2': [f1],
+             },
+            results,
+        )
+
     def test_collapse(self):
         files = []
         self._make_python_package()