Incorrect warning about local commits

[jakubbortlik]

Prerequsities

• I'm on the latest version of the plugin
• I've installed the required dependencies
• I've run :h gitlab.nvim.troubleshooting and followed the steps there

Bug Description

The warning message You have local commits that are not on origin. Have you forgotten to push? is shown incorrectly in at least the following cases:

• The MR has already been closed and the feature branch has been deleted - in this case the warning is completely misleading.
• The feature branch has been rebased on the remote - in this case the message could rather say "Have you forgotten to pull?".

The gitlab.git.current_branch_up_to_date_on_remote function should be changed so that it is more universal, ideally without making it slower by adding more git calls.

[harrisoncramer]

This is addressed here, please take a look: #395