Impl/stash selected changes

[MS-Painter]

Closes #11531

Description

Resolving this issue revealed underlying issues in Desktop's current implementation and handling of git stashes that affected the solution I went with:

• Desktop only recognizes and manages git stashes made by Desktop.
• Desktop doesn't fully manage all of a branch's stashes.
• git stash commands can't "ammend" existing stashes.

Given these issues, there is no currently defined right approach that Desktop adheres to.
Leading me to decide to try to solve the issue from the user experience POV the issue itself expected -
A singular stash Desktop is familiar with that can be ammended.

The actual implementation I went with:

• "Ammend" existing Desktop stash by using git stash pop then pushing back with the new changes.
• Stash selected files (does not include diff partial file stashes).

Screenshots

Release notes

Notes:

[MS-Painter]

An issue I'm aware of that needs cleanup -
When we do the temporary git stash pop we apply _refreshRepository which makes the UI show the popped files briefly.

git.stash.pop.issue.mp4

[jorisw]

@tidy-dev Please either review or close this nearly year old PR.