Revert "Merge branch 'make-builtin-stash-and-rebase-opt-ins'"

[derrickstolee]

This reverts commit cec98c6, reversing
changes made to 0bccc7f.

This removes the builtin rebase/stash operations until we can figure out how to make them work well with post-command hooks.

[derrickstolee]

@dscho This reverts the builtin stash/rebase stuff because it interacts badly with the post-command hooks (on Mac). At this point, it is probably safest to revert it for the next release and harden this interaction before the next release. @jrbriggs thoughts?

[dscho]

Shouldn't we rather change the sane_execvp() call by something like exit(run_command_v_opt(...))? (Same in stash.c)

[derrickstolee]

Abandoning in favor of #36