dashboard: skip x/review on plan9 builders

Plan 9 has a non-standard "git" CLI,
and x/review contains only a git hook.

Change-Id: I9b77ae772afa714e95d814b038a4ebedbd95151c
Reviewed-on: https://go-review.googlesource.com/c/build/+/368154
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: David du Colombier <0intro@gmail.com>
diff --git a/dashboard/builders.go b/dashboard/builders.go
index c7a12af..e07d0f2 100644
--- a/dashboard/builders.go
+++ b/dashboard/builders.go
@@ -3048,6 +3048,10 @@
 // both filesystem-intensive and unlikely to be relevant to plan9 users.
 func plan9Default(repo, branch, goBranch string) bool {
 	switch repo {
+	case "review":
+		// The x/review repo tests a Git hook, but the plan9 "git" doesn't have the
+		// same command-line API as "git" everywhere else.
+		return false
 	case "website":
 		// The x/website tests read and check the website code snippets,
 		// which require many filesystem walk and read operations.