Expose alternative strategies for resolving uncommitted changes

[hadley]

e.g. this:

There are uncommitted changes, which may cause problems or be lost when we push, pull, switch, or compare branches.
Do you want to proceed anyway?
1: Nope
2: Yes
3: No way

Could instead be:

What do you want to do?
1. Stash changes (restore with git_stash_pop())
2. Abort this operation
3. Try again, I've done something with git
4. Proceed anyway

[jennybc]

My initial thought is that, if we expose stashing, that should include interactive support for popping, similar to pr_resume() or pr_fetch(), showing you the most likely suspects and, probably, which branch they were stashed on.