Allow stashing changes even if stash is already present but there is no conflict

[Andrej730]

The feature request

Let's say you edited file A and then stashed the changes.
Then you either added other changes to A or modified file B and if you try to stash you'll get a warning previous stash will be discarded.

Current workaround is to restore stashed changes (it's done in clever way and works fine if there are unstaged changes without conflicts) and then stash them all.

Proposed solution

Allow adding new changes to stash if there are no conflicts.

Additional context

This feature will be very useful if #11531 will be implemented.

[justingolden21]

+1

Have been thinking this for a while but never bothered to make an issue.