Add "revert" command

`topydo revert` will search for backup corresponding with current state
of todo file, and use it to recover previous state. Output will show to
user results of which command had been reverted. If no suitable backup
is found (for example if user edited todo file recently with other
application), no action is taken and user is notified on the output
about lack of corresponding backup in backup file.

topydo/Commands.py

@@ -47,6 +47,7 @@ _SUBCOMMAND_MAP = {
     'postpone': 'PostponeCommand',
     'pri': 'PriorityCommand',
     'quit': 'ExitCommand',
+    'revert': 'RevertCommand',
     'rm': 'DeleteCommand',
     'sort': 'SortCommand',
     'tag': 'TagCommand',

topydo/cli/CLIApplicationBase.py

@@ -57,6 +57,7 @@ Available commands:
 * listprojects (lsprj)
 * postpone
 * pri
+* revert
 * sort
 * tag
 
@@ -200,7 +201,7 @@ class CLIApplicationBase(object):
         Execute a subcommand with arguments. p_command is a class (not an
         object).
         """
-        cmds_wo_backup = tuple(cmd + 'Command' for cmd in READ_ONLY_COMMANDS
+        cmds_wo_backup = tuple(cmd + 'Command' for cmd in ('Revert', ) + READ_ONLY_COMMANDS)
         if config().backup_count() > 0 and p_command and not p_command.__module__.endswith(cmds_wo_backup):
             call = [p_command.__module__.lower()[16:-7]] + p_args # strip "topydo.commands" and "Command"
             self.backup = ChangeSet(self.todolist, p_call=call)

topydo/commands/RevertCommand.py

+# Topydo - A todo.txt client written in Python.
+# Copyright (C) 2014 - 2015 Bram Schoenmakers <me@bramschoenmakers.nl>
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+from topydo.lib.Command import Command
+from topydo.lib.ChangeSet import ChangeSet
+from topydo.lib import TodoFile
+from topydo.lib import TodoList
+from topydo.lib.Config import config
+
+class RevertCommand(Command):
+    def __init__(self, p_args, p_todolist,
+                 p_out=lambda a: None,
+                 p_err=lambda a: None,
+                 p_prompt=lambda a: None):
+        super(RevertCommand, self).__init__(
+            p_args, p_todolist, p_out, p_err, p_prompt=lambda a: None)
+
+    def execute(self):
+        if not super(RevertCommand, self).execute():
+            return False
+
+        archive_file = TodoFile.TodoFile(config().archive())
+        archive = TodoList.TodoList(archive_file.read())
+
+        last_change = ChangeSet()
+
+        try:
+            last_change.get_backup(self.todolist)
+            last_change.apply(self.todolist, archive)
+            archive_file.write(archive.print_todos())
+            last_change.delete()
+
+            self.out("Successfully reverted: " + last_change.call)
+        except (ValueError, KeyError):
+            self.error('No backup was found for the current state of ' + config().todotxt())
+
+        last_change.close()
+
+
+    def usage(self):
+        return """Synopsis: revert"""
+
+    def help(self):
+        return """\
+Reverts the last command.
+"""