my_activity.py: Add date to format

Allows adding the created and modified dates to the output. Doesn't
change the default behavior.

Example:
$ my_activity.py --begin=01/01/19 -f "{created} {modified} {url} {title}"
2018-12-19 2019-01-09 https://crbug.com/916521 Tests do not resolve libbrillo-libchrome dependency properly
2018-12-05 2019-01-14 https://crbug.com/912312 authpolicy: Cache authentication data

BUG=None

Change-Id: Idfe8aa9553dfb8f9774ee4915e85b6167e6fb75b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1511252
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>

my_activity.py

@@ -595,6 +595,8 @@ class MyActivity(object):
                        change['header'],
                        change['review_url'],
                        change['author'],
+                       change['created'],
+                       change['modified'],
                        optional_values)
 
   def print_issue(self, issue):
@@ -609,6 +611,8 @@ class MyActivity(object):
                        issue['header'],
                        issue['url'],
                        issue['author'],
+                       issue['created'],
+                       issue['modified'],
                        optional_values)
 
   def print_review(self, review):
@@ -631,11 +635,13 @@ class MyActivity(object):
                        review['header'],
                        review['review_url'],
                        review['author'],
+                       review['created'],
+                       review['modified'],
                        optional_values)
 
   @staticmethod
   def print_generic(default_fmt, specific_fmt,
-                    title, url, author,
+                    title, url, author, created, modified,
                     optional_values=None):
     output_format = specific_fmt if specific_fmt is not None else default_fmt
     output_format = unicode(output_format)
@@ -643,6 +649,8 @@ class MyActivity(object):
         'title': title,
         'url': url,
         'author': author,
+        'created': created,
+        'modified': modified,
     }
     if optional_values is not None:
       values.update(optional_values)
@@ -930,9 +938,10 @@ def main():
                               'individually for each activity type. The format '
                               'is defined as documented for '
                               'string.format(...). The variables available for '
-                              'all activity types are url, title and author. '
+                              'all activity types are url, title, author, '
-                              'Format options for specific activity types will '
+                              'created and modified. Format options for '
-                              'override the generic format.')
+                              'specific activity types will override the '
+                              'generic format.')
   output_format_group.add_option(
       '-f', '--output-format', metavar='<format>',
       default=u'{url} {title}',