reset.c: share call to die_if_unmerged_cache()

reset.c: share call to die_if_unmerged_cache()

Use a single condition to guard the call to die_if_unmerged_cache for
both --soft and --keep. This avoids the small distraction of the
precondition check from the logic following it.

Also change an instance of

  if (e)
    err = err || f();

to the almost as short, but clearer

  if (e && !err)
    err = f();

(which is equivalent since we only care whether exit code is 0)

Signed-off-by: Martin von Zweigbergk <martinvonz@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>