cmd/go: check test cache more efficiently

[ijt]

Fixes #26726

Before this change, the same hashes were being computed multiple times
in some cases.

This gets the case reported in issue #26726 down from over 30s to .8s.

[gopherbot]

Message from Gerrit User 5976:

Patch Set 1:

Congratulations on opening your first change. Thank you for your contribution!

Next steps:
Within the next week or so, a maintainer will review your change and provide
feedback. See https://golang.org/doc/contribute.html#review for more info and
tips to get your patch through code review.

Most changes in the Go project go through a few rounds of revision. This can be
surprising to people new to the project. The careful, iterative review process
is our way of helping mentor contributors and ensuring that their contributions
have a lasting impact.

During May-July and Nov-Jan the Go project is in a code freeze, during which
little code gets reviewed or merged. If a reviewer responds with a comment like
R=go1.11, it means that this CL will be reviewed as part of the next development
cycle. See https://golang.org/s/release for more details.

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/127155.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Gerrit User 10961:

Patch Set 1:

(1 comment)

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/127155.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Gerrit User 28552:

Patch Set 1:

(1 comment)

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/127155.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Gerrit User 12446:

Uploaded patch set 3: New patch set was added with same tree, parent, and commit message as Patch Set 2.

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/127155.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Gerrit User 10961:

Patch Set 3:

(2 comments)

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/127155.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Gerrit User 28552:

Patch Set 3:

(1 comment)

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/127155.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Gerrit User 28552:

Patch Set 3:

I got good results by removing the EvalSymlinks calls from inDir as suggested by rsc in #26562. I'm thinking it will be best to revert this change and make a new one.

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/127155.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

This PR is being closed because golang.org/cl/127155 has been abandoned.