import-all() support git submodule and add drop-all() function

[allenyllee]

Hi @501st-alpha1 ,
I come from your stackoverflow anwser, and tries to use your script, it's a great job! But I've encountered some problems, so I try to fix those as below:

1. import-all() function support git submodule
2. fix import() function prompt error when its first time stash commit
3. push() and drop() function use full length SHA instead of short SHA (because I've encounter a problem of different short SHA length between machines)
4. Also delete local refs/stashes when drop() remote refs/stashes
5. add drop-all() function

[501st-alpha1]

Hi @allenyllee! Thank you for the contributions! I'm glad this script was useful to you, and I appreciate the fixes and improvements.

One request I have before I merge these changes in is could you please split these changes into a few separate commits (and/or PRs)? I prefer to have smaller, atomic commits so that each change can be examined individually. Probably one commit for each of your five items would be okay, and I'd be fine with either a new PR for each one, or maybe combine the commits for 1 - 3 into one PR since those are all bugfixes, and then have a separate PR each for 4 and 5.

You may also find this post about Git commit messages helpful. Let me know if you have any questions about anything I've said, or about the technical details of how to actually split this up into multiple commits; I'm happy to help.

Thanks again for taking the time to contribute!