read base commit hash when worktree is opened

set base commit to invalid commit id when work tree is created

check read() return value properly in read_meta_file()

rename variable gotpath to path_got

add a proper definition for the worktree head

create an emtpy base-commit file

add verbose output to worktree_test -v

fix mem leak in worktree test code

in work tree test code, use stat() to test existence of file

plug file descriptor leak in worktree test code

fail test if worktree cannot be removed

rename worktree test helper function

introduce a helper function for use by work tree init test

use a dedicated file for the work tree lock

document how work tree locking will work