setup: update the right file in multiple checkouts
This code is introduced in
23af91d (prune: strategies for linked
checkouts - 2014-11-30), and it's supposed to implement this rule from
that commit's message:
- linked checkouts are supposed to keep its location in $R/gitdir up
to date. The use case is auto fixup after a manual checkout move.
Note the name, "$R/gitdir", not "$R/gitfile". Correct the path to be
updated accordingly.
While at there, make sure I/O errors are not silently dropped.
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>