fix: stash summary wasn't showing correctly

[wenfangdu]

Fixes #91.

[wenfangdu]

Before:

After:

The side effect is that the branch name is gone.

[wenfangdu]

When dropping a stash, there's no need to show a notification, removing the stash entry from the panel is clear enough.
There're a lot of improvements that can be done for this extension, such as stash renaming, showing relative stash date, etc. https://github.com/gitkraken/vscode-gitlens is a good codebase to refer to.