revision.c: better error reporting on ref from different worktrees

revision.c: better error reporting on ref from different worktrees

Make use of the new ref aliases to pass refs from another worktree
around and access them from the current ref store instead. This does
not change any functionality, but when a problem arises, we would like
the reported messages to mention full ref aliases, like this:

    fatal: bad object worktrees/ztemp/HEAD
    warning: reflog of 'main-worktree/HEAD' references pruned commits

instead of

    fatal: bad object HEAD
    warning: reflog of 'HEAD' references pruned commits

which does not really tell where the refs are from.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>