pull: use opts.get('bookmark') instead of opts['bookmark']

Authored by pulkit.

Description

pull: use opts.get('bookmark') instead of opts['bookmark']

This is done because at places in hgsubversion, we call the function directly. I
expect there might be more instances in extensions out there which calls
commands.push() directly. So let's not require explicitly passing of bookmark
value.

The use of opts['bookmark'] was introduced in
bad05a6afdc89cc58a2af320698ab29bd8de62d4.

Differential Revision: https://phab.mercurial-scm.org/D5484