Add support for release dates

- Adds a new config option (default is false)
- Adds ability to go from a tag to a date

Change-Id: I14c566dc6b0220999dd803be5c7374c05acb4748

releasenotes/notes/release-date-3a1dec42c91a3f0b.yaml

@@ -0,0 +1,5 @@
+---
+features:
+  - |
+    Add an option, ``add_release_date``, to print the release dates for every
+    version.

reno/config.py

@@ -64,6 +64,12 @@ _OPTIONS = [
         The template used by reno new to create a note.
         """)),
 
+    Opt('add_release_date', False,
+        textwrap.dedent("""\
+        Should the report include release date (True) based on
+        the date of objects associated with the release tag.
+        """)),
+
     Opt('release_tag_re',
         textwrap.dedent('''\
         ((?:v?[\\d.ab]|rc)+)  # digits, a, b, and rc cover regular and

reno/formatter.py

@@ -11,6 +11,7 @@
 # under the License.
 
 from __future__ import print_function
+from datetime import datetime
 
 
 def _indent_for_list(text, prefix='  '):
@@ -71,6 +72,15 @@ def format_report(loader, config, versions_to_include, title=None,
         report.append('=' * len(version_title))
         report.append('')
 
+        if (config.add_release_date
+                # Currently, this only works when the Scanner is used
+                and loader._scanner is not None
+                and version in loader._scanner._repo._tags_to_dates.keys()):
+            date = datetime.fromtimestamp(
+                loader._scanner._repo._tags_to_dates[version])
+            report.append('Release Date: ' + date.strftime("%Y-%m-%d"))
+            report.append('')
+
         # Add the preludes.
         notefiles = loader[version]
         prelude_name = config.prelude_section_name

reno/scanner.py

@@ -391,6 +391,7 @@ class RenoRepo(repo.Repo):
     # Populated by _load_tags().
     _all_tags = None
     _shas_to_tags = None
+    _tags_to_dates = None
 
     def _get_commit_from_tag(self, tag, tag_sha):
         """Return the commit referenced by the tag and when it was tagged."""
@@ -434,9 +435,11 @@ class RenoRepo(repo.Repo):
             if k.startswith(b'refs/tags/')
         }
         self._shas_to_tags = {}
+        self._tags_to_dates = {}
         for tag, tag_sha in self._all_tags.items():
             tagged_sha, date = self._get_commit_from_tag(tag, tag_sha)
             self._shas_to_tags.setdefault(tagged_sha, []).append((tag, date))
+            self._tags_to_dates[tag] = date
 
     def get_tags_on_commit(self, sha):
         "Return the tag(s) on a commit, in application order."