Merge branch 'ab/checkout-branch-info-leakfix'

We added an unrelated sanity checking that leads to a BUG() while
plugging a leak, which triggered in a repository with symrefs in
the local branch namespace that point at a ref outside.  Partially
revert the change to avoid triggering the BUG().

* ab/checkout-branch-info-leakfix:
  checkout: avoid BUG() when hitting a broken repository

diff --cc builtin/checkout.c
Simple merge

diff --cc t/t2018-checkout-branch.sh
Simple merge