cmd/go/internal/modfetch/codehost: work around an apparent bug in 'git fetch --unshallow'

When 'git fetch' is passed the '--unshallow' flag, it assumes that the
local and remote refs are equal.¹ However, we were fetching an
expanded set of refs explicitly in the same command, violating that
assumption.

Now we first expand the set of refs, then unshallow the repo in a
separate fetch. Empirically, this seems to work, whereas the opposite
order does not.

¹https://github.com/git/git/blob/4c86140027f4a0d2caaa3ab4bd8bfc5ce3c11c8a/transport.c#L1303-L1309

Fixes #34266

Change-Id: Ie97eb7c1223f944003a1e31d0ec9e69aad0efc0d
Reviewed-on: https://go-review.googlesource.com/c/go/+/196961


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>