Fix - stashlist removes marked commit IDs from state after stash drop

[lightsnowball]

This Pull Request fixes issue related to dropping multiple stashes (no issue number as far as I can tell). Once we drop marked stash, GitUI's state becomes invalid and the user is not able to drop/pop stash anymore. The scenario is covered in the gif below (after dropping, there remains 1 stash commit in the marked commits state).

It changes the following:
Fixes the described problem.

I followed the checklist:

• I added unittests
• I ran make check without errors
• I tested the overall application
• I added an appropriate item to the changelog

I'll add unittests (not sure how should I test it tbh) and changelog later if this fix makes sense.

[extrawurst]

thanks for discovering the bug, your solution to fix it was way too complicated for me to understand. please don't take this personally and use the opportunity to check how I fixed it in #1207