read-cache: move shared commit and ls-files code

read-cache: move shared commit and ls-files code

The function overlay_tree_on_index(), plus associated helper functions,
were defined in builtin/ls-files.c, but also shared with
builtin/commit.c.  Move these shared functions to read-cache.c.

Diff best viewed with `--color-moved`.

Signed-off-by: Elijah Newren <redacted>
Signed-off-by: Junio C Hamano <redacted>