update txtpbfmt to drop indirect dependency on glog

cuelang.org/go/encoding/protobuf/textproto imports
github.com/protocolbuffers/txtpbfmt/parser,
which is a fairly low-level package but offers optional log messages.

Those used to be implemented directly with github.com/golang/glog,
meaning that CUE users had to include glog as an indirect dependency.
This has minor implications in terms of download and binary sizes,
but it also meant that glog's global flags got registered in package
flag's default flagset, potentially affecting Go library users as well.

We sent a couple of pull requests to txtpbfmt to replace its hard
dependency on glog in favor of a minimal logging interface.
Now that those are merged, we can update the version.
One can see that glog is no longer an indirect requirement in go.mod.

The full list of changes included in upstream's 20 commits is at:
https://github.com/protocolbuffers/txtpbfmt/compare/74888fd59c2b...3462fbc510c0
Of note is that the parser preserves more comments now,
which causes a minor improvement in our test cases.

Fixes #1199.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I2c14a12e343f748df6e66b390495d72f628a96c4
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/552359
Unity-Result: CUEcueckoo <cueckoo@cuelang.org>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Paul Jolly <paul@myitcv.io>
Reviewed-by: Marcel van Lohuizen <mpvl@gmail.com>