fetch: add top-level trace2 regions

fetch: add top-level trace2 regions

At $DAYJOB we experienced some slow fetch operations and needed some
additional data to help diagnose the issue.

Add top-level trace2 regions for the various modes of operation of
`git-fetch`. None of these regions are in recursive code, so any
enclosed trace messages should only see their nesting level increase by
one.

Signed-off-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>