stash: introduce 'git stash store'

save_stash() contains the logic for doing two potentially independent
operations; the first is preparing the stash merge commit, and the
second is updating the stash ref/ reflog accordingly.  While the first
operation is abstracted out into a create_stash() for callers to access
via 'git stash create', the second one is not.  Fix this by factoring
out the logic for storing the stash into a store_stash() that callers
can access via 'git stash store'.

Like create, store is not intended for end user interactive use, but for
callers in other scripts.  We can simplify the logic in the
rebase.autostash feature using this new subcommand.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
rebase: use 'git stash store' to simplify logic

rebase has no reason to know about the implementation of the stash.  In
the case when applying the autostash results in conflicts, replace the
relevant code in finish_rebase () to simply call 'git stash store'.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>