cmd/go: skip tests using Git on Plan 9

TestScript/get_unicode, TestScript/get_dotfiles and
TestScript/get_brace are failing on Plan 9 since they
expect a full-featured git command, while the git tool
has been emulated as a simple rc script on Plan 9.

This change skips tests using Git on Plan 9.

Fixes #29640.

Change-Id: Id7f6fdca552167f4631fe401f63167e5653daafa
Reviewed-on: https://go-review.googlesource.com/c/157119
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/script_test.go

@@ -283,6 +283,11 @@ Script:
 				if strings.HasPrefix(cond, "exec:") {
 					prog := cond[len("exec:"):]
 					ok = execCache.Do(prog, func() interface{} {
+						if runtime.GOOS == "plan9" && prog == "git" {
+							// The Git command is usually not the real Git on Plan 9.
+							// See https://golang.org/issues/29640.
+							return false
+						}
 						_, err := exec.LookPath(prog)
 						return err == nil
 					}).(bool)