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

[Ralf Jung <post@ralfj.de>]

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