Use remote of current branch in push/pull/fetch dialogs

This makes repeated pushes of the same branch to the same remote easier.
The user can do it from the keyboard with Ctrl+P, return.
Merge pull request #324 from takluyver/dialogs-default-remote

Use remote of current branch in push/pull/fetch dialogs

Signed-off-by: David Aguilar <davvid@gmail.com>