Merge branch 'rb/merge-prepare-commit-msg-hook' into pu

Merge branch 'rb/merge-prepare-commit-msg-hook' into pu

Expose more merge states (e.g. $GIT_DIR/MERGE_MODE) to hooks that
run during "git merge".  The log message stresses too much on one
hook, prepare-commit-msg, but it would equally apply to other hooks
like post-merge, I think.

* rb/merge-prepare-commit-msg-hook:
  merge: drop unused arg from abort_commit method signature
  merge: make prepare_to_commit responsible for write_merge_state
  t7505: ensure cleanup after hook blocks merge
  t7505: add missing &&

# Conflicts:
# 	builtin/merge.c
# 	t/t7505-prepare-commit-msg-hook.sh