gitcmds: remove unnecessary list() calls

The list.extend() method takes arbitrary iterables, so pass in set
objects directly instead of converting them to lists first.

Signed-off-by: Daniel Harding <dharding@living180.net>
gitcmds: don't munge modified_submods set

Previously, the worktree_state_dict() function would remove any elements
from the modified_submods set that were in the staged_submods set.
However, this would give incorrect results if a submodule really had
staged and unstaged changes.  Stop munging the modified_submods set to
allow the actual state to be seen in the status widget.

Signed-off-by: Daniel Harding <dharding@living180.net>
Merge pull request #379 from living180/submodules_tweaks

Tweaks to worktree_state_dict()

Signed-off-by: David Aguilar <davvid@gmail.com>