feat(ops/besadii): Make branch key cl/XXXX

The branch key for buildkite builds isn't actually used to fetch if a
commit is given - instead, it's just a visual grouping of multiple
builds. This means we can just make the branch key cl/<cl number>, which
is the convention we already use to refer to CLs and gets us a nice
visual grouping of builds of successive patchsets of the same CL number,
even though the ref we're providing isn't a real ref.

Change-Id: Iaa9111297a88f965fda94cd8266240106f58a100
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4347
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>
Autosubmit: grfn <grfn@gws.fyi>

ops/besadii/main.go

@@ -28,6 +28,7 @@ import (
 	"path"
 	"regexp"
 	"strconv"
+	"strings"
 )
 
 // Regular expression to extract change ID out of a URL
@@ -202,9 +203,12 @@ func triggerBuild(cfg *config, log *syslog.Writer, trigger *buildTrigger) error
 		headBuild = false
 	}
 
+    // The branch doesn't have to be a real ref (it's just used to group builds), so make it the identifier for the CL
+	branch := fmt.Sprintf("cl/%v", strings.Split(trigger.ref, "/")[3])
+
 	build := Build{
 		Commit: trigger.commit,
-		Branch: trigger.ref,
+		Branch: branch,
 		Env:    env,
 		Author: Author{
 			Name:  trigger.author,