Merge branch 'jc/maint-co-track' into maint

* jc/maint-co-track:
Enhance hold_lock_file_for_{update,append}() API
demonstrate breakage of detached checkout with symbolic link HEAD
Fix "checkout --track -b newbranch" on detached HEAD

diff --cc builtin-revert.c
Simple merge

diff --cc sha1_file.c
Simple merge

diff --cc t/t7201-co.sh
Simple merge