branch: add branch_checked_out() helper

branch: add branch_checked_out() helper

The validate_new_branchname() method contains a check to see if a branch
is checked out in any non-bare worktree. This is intended to prevent a
force push that will mess up an existing checkout. This helper is not
suitable to performing just that check, because the method will die()
when the branch is checked out instead of returning an error code.

Create a new branch_checked_out() helper that performs the most basic
form of this check. To ensure we can call branch_checked_out() in a loop
with good performance, do a single preparation step that iterates over
all worktrees and stores their current HEAD branches in a strmap. The
branch_checked_out() helper can then discover these branches using a
hash lookup.

This helper is currently missing some key functionality. Namely: it
doesn't look for active rebases or bisects which mean that the branch is
"checked out" even though HEAD doesn't point to that ref. This
functionality will be added in a coming change.

We could use branch_checked_out() in validate_new_branchname(), but this
missing functionality would be a regression. However, we have no tests
that cover this case!

Add a new test script that will be expanded with these cross-worktree
ref updates. The current tests would still pass if we refactored
validate_new_branchname() to use this version of branch_checked_out().
The next change will fix that functionality and add the proper test
coverage.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>