Add config option to make "--keep-index" the default for "git stash push"

Hi all,

I have been repeatedly bitten by the default behavior of `git stash` to stash not just the unstaged but also the staged changes, and then `git stash pop` making all changes unstaged. This means `git stash && git stash pop`, per default, loses information, and when I just spent 10min carefully selecting hunks to be staged that's quite frustrating.

I found that for myself, I usually expect `git stash` to only stash the changes that are not yet staged. So I'd like to configure git such that `--keep-index` is the default. That would also fix the information loss I mentioned above. (By now I am also aware of `git stash pop --index`, but (a) that's not the default either and (b) its documentation indicates it might not always work very well.) However, going over the `git-config` man page, I have not found any way to change the default behavior. Is that possible somehow? And if not, could you consider this a feature request to add such an option?

Kind regards, Ralf