Revert "main.model: remove redundant function"

This reverts commit 83d18826c935e03eb3cd8f16a9f669231ff0f3a0.

This function is actually not redundant... it allows
us to unstage while amending.  Add a comment.

Signed-off-by: David Aguilar <davvid@gmail.com>
main.model: add a docstring to unstage_all()

Signed-off-by: David Aguilar <davvid@gmail.com>