refresh_index: rename format variables

refresh_index: rename format variables

When refreshing the index, for modified (or unmerged) files we will print
"needs update" (or "needs merge") for plumbing, or line similar to the
output from "diff --name-status" for porcelain.

The variables holding which type of message to show are named after the
plumbing messages. However, as we begin to differentiate more cases at the
porcelain level (with the plumbing message staying the same), that naming
scheme will become awkward.

Instead, name the variables after which case we found (modified or
unmerged), not what we will output.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>