internal/ci/gerrithub: update comment to be clearer

The script does not replace slashes with underscores,
which is what the comment seems to suggest is intended.
All the script does is keep the last two path elements.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I2a8a95d54fc9b09711e7f346967fe59e2ac37aca
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/551167
Reviewed-by: Paul Jolly <paul@myitcv.io>
Unity-Result: CUEcueckoo <cueckoo@cuelang.org>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>

internal/ci/gerrithub/gerrithub.cue

@@ -65,8 +65,9 @@ _#linuxMachine: "ubuntu-20.04"
 			if:        "${{ github.event.client_payload.type == '\(#type)' }}"
 			steps: [
 				#writeNetrcFile,
-				// Hack to get the ref (e.g. refs/changes/38/547738/7) in a format we can use in a
-				// branch name, e.g. _547738_7
+				// Out of the entire ref (e.g. refs/changes/38/547738/7) we only
+				// care about the CL number and patchset, (e.g. 547738/7).
+				// Note that gerrithub_ref is two path elements.
 				json.#step & {
 					id: "gerrithub_ref"
 					run: #"""