sequencer: rename reset_for_rollback to reset_merge

sequencer: rename reset_for_rollback to reset_merge

We are on a path to teach cherry-pick/revert how to skip commits. To
achieve this, we could really make use of existing functions.
reset_for_rollback is one such function, but the name does not
intuitively suggest to use it to reset a merge, which it was born to
perform, see 539047c ("revert: introduce --abort to cancel a failed
cherry-pick", 2011-11-23). Change the name to reset_merge to make
it more intuitive.

Signed-off-by: Rohit Ashiwal <rohit.ashiwal265@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>