create filetime_from_hg plugin

this is mostly an adapted copy of filetime_from_git to target mercurial
repositories.

filetime_from_hg/README.rst

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+Use Mercurial commit to determine page date
+===========================================
+
+If your blog content is versioned via Mercurial, this plugin will set
+articles' and pages' ``metadata['date']`` to correspond to that of the
+hg commit.  This plugin depends on the ``hglib`` python package,
+which can be installed via::
+
+    sudo apt-get install python-hglib
+
+or::
+
+    pip install hglib
+
+The date is determined via the following logic:
+
+* if a file is not tracked by hg, or a file is added but never committed
+    - metadata['date'] = filesystem time
+    - metadata['updated'] = filesystem time
+* if a file is tracked, but no changes in working directory
+    - metadata['date'] = first commit time
+    - metadata['updated'] = last commit time
+* if a file is tracked, and has changes in working directory
+    - metadata['date'] = first commit time
+    - metadata['updated'] = filesystem time
+
+When this module is enabled, ``date`` and ``updated`` will be determined
+by hg status; no need to manually set in article/page metadata. And
+operations like copy and move will not affect the generated results.
+
+If you don't want a given article or page to use the hg time, set the
+metadata to ``hgtime: off`` to disable it.
+
+You can also set ``HG_FILETIME_FOLLOW`` to ``True`` in your settings to
+make the plugin follow file renames — i.e., ensure the creation date matches
+the original file creation date, not the date it was renamed.
+
+Credits
+=======
+
+This plugin is based on filetime_from_git.

filetime_from_hg/__init__.py

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+from .filetime_from_hg import *

filetime_from_hg/filetime_from_hg.py

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+# -*- coding: utf-8 -*-
+"""
+This plugin is mostly a copy of the filetime_from_git plugin
+by Zhang Cheng <StephenPCG@gmail.com> and others.
+
+Copyright (c) David Douard <david.douard@sdfa3.org>
+
+Compute Date and Modified metadata from Mercurial revisions.
+"""
+
+import os
+from pelican import signals, contents
+from pelican.utils import strftime, set_date_tzinfo
+from datetime import datetime
+
+import hglib
+
+def datetime_from_timestamp(timestamp, content):
+    """
+    Helper function to add timezone information to datetime,
+    so that datetime is comparable to other datetime objects in recent versions
+    that now also have timezone information.
+    """
+    return set_date_tzinfo(
+        datetime.fromtimestamp(timestamp),
+        tz_name=content.settings.get('TIMEZONE', None))
+
+
+def filetime_from_hg(content):
+    if isinstance(content, contents.Static):
+        return
+    if 'date' in content.metadata:
+        # if user did explicitely set a date, do not overwrite it
+        return
+    repo = hglib.open('.')
+    tz_name = content.settings.get('TIMEZONE', None)
+
+    hgtime = content.metadata.get('hgtime', 'yes').lower()
+    if hgtime in ('no', 'off', 'false', '0'):
+        return
+
+    # 1. file is not managed by hg
+    #    date: fs time
+    # 2. file is staged, but has no commits
+    #    date: fs time
+    # 3. file is managed, and clean
+    #    date: first commit time, update: last commit time or None
+    # 4. file is managed, but dirty
+    #    date: first commit time, update: fs time
+    path = content.source_path
+    root = repo.root()
+    filelog = repo.log(revrange='.:0', files=[path,],
+                       follow=content.settings.get('HG_FILETIME_FOLLOW', False))
+    if filelog:
+        # has commited
+        content.date = set_date_tzinfo(filelog[-1][6], tz_name)
+        if path in [os.path.join(root, mfile) for flag, mfile in repo.status(modified=True)]:
+            # file is modified in the wd
+            content.modified = datetime_from_timestamp(
+                os.stat(path).st_ctime, content)
+        else:
+            # file is not changed
+            if len(filelog) > 1:
+                content.modified = set_date_tzinfo(filelog[0][6], tz_name)
+    else:
+        # file is not managed by hg
+        content.date = datetime_from_timestamp(os.stat(path).st_ctime, content)
+
+    if not hasattr(content, 'modified'):
+        content.modified = content.date
+
+    content.locale_date = strftime(content.date, content.date_format)
+    content.locale_modified = strftime(content.modified, content.date_format)
+    # ensure the metadata directory is synchronized. Might be used by
+    # some other plugin (eg. series)
+    content.metadata['modified'] = content.modified
+    content.metadata['date'] = content.date
+
+
+def register():
+    signals.content_object_init.connect(filetime_from_hg)