Provide similar functionality to 'git' command-line tool in 'dulwich' command

[jelmer]

It would be great if the functionality provided by the 'dulwich' command-line tool was equivalent to that provided by the C git "git" command-line tool.

[RobJamesRamos]

It would be great if the 'dulwich' command had more overlap with 'git' commands. A merge command seems like the most glaring omission at the moment.

[jelmer]

Help with this would be great. See #452 about a merge implementation.

[suliveevil]

Less dependencies is always better!