Better handling of erroneous git stash save "somemessage" --keep-index

Hi,

the command:

git stash save "some message" --keep-index

stashes everything, including the index, and adds the "--keep-index"
to the message. The manual labor of separating index hunks from hunks
to stash is lost. This is in version 1.8.1.3.

This is a user error, of course (per the man page, parameters are not
accepted after the message). But it would be better handled by
erroring out, with a message like "git stash save does not permit
parameters after the stash message".

Cheers,

Gulli