cmd/go: copy git reuse and support repos to hg

The reuse_hg.txt is reuse_git.txt with a skip at the top
and a global substitute of git->hg and fetch->pull.

The prefixtagtests.txt and tagtests.txt are straight
copies of the git equivalents.

This is to set up a readable diff in the followup CL
that turns it into a reuse test for hg.

For #75119.

Change-Id: I50c7f5559ee6479f8328ed54c14eafcc7f1ecc1f
Reviewed-on: https://go-review.googlesource.com/c/go/+/718381
Reviewed-by: Michael Matloob <[email protected]>
Reviewed-by: Michael Matloob <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: rsc