Factor the functions out, so they can be re-used from other places. In
particular these functions will be re-used in builtin/worktree.c to make
git worktree add dwim more.
While there add some docs to the function.
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
