testscript: make testlog work with custom commands

[mvdan]

See https://golang.org/src/testing/internal/testdeps/ - this code is used so that go test knows what files to include when computing the test cache key.

All the files that the test binary directly opens are included in said cache key. This includes the testscript txt files.

However, custom commands are run by doing nested execs of the test binary. The files opened by the nested process alone are thus not included in the testlog file.

For example, in this example above, /foo/bar in the git repository would be consumed by a test script, but it wouldn't be included in the test cache key. If modified, go test would happily not re-run the tests.

my-cmd read-file ${GIT_ROOT}/foo/bar

I think we should add some code to help the magic work with nested exec calls, similar to what we did for code coverage.

/cc @rogpeppe @myitcv