Some codepaths, including the refs API, get and keep relative
paths, that go out of sync when the process does chdir(2). The
chdir-notify API is introduced to let these codepaths adjust these
cached paths to the new current directory.
* jk/relative-directory-fix:
refs: use chdir_notify to update cached relative paths
set_work_tree: use chdir_notify
add chdir-notify API
trace.c: export trace_setup_key
set_git_dir: die when setenv() fails