How to reproduce:
1. Create a new project
2. Open the Package Manager window
3. Click the '+' button to add a package from git URL
4. Enter any valid Unity package repository link (for example, create a git repository with the attached test-unity-package.zip)

Expected result: a progress indicator is shown until the import begins
Actual result: no progress indicator is shown and the Editor seems idle until the import begins

Reproducible with: 2019.4.0f1, 2020.1.0b12, 2020.2.0a14
Could not test with: 2018.4.23f1 (adding packages from git not introduced)

Notes:
1. This is also an issue when importing the same package twice as there's no progress indicator and no import happens, so the command might seem to not be working at all.
2. The 'idle' time sometimes can be longer than shown in the attached video (if the internet connection is slower)