zip: skip tests that run 'git init' and 'git config' on plan9

The plan9 'git' command empirically doesn't support one or more of the
commands we need to run to create a test repository
(https://build.golang.org/log/627dacbfca9e0dfe6b84cf597411e63898b68ec8).
Rather than trying to coax it into working with a non-standard git
command, just skip the tests that need it.

Change-Id: Id20fd21d4a4b9d2d683ba7bdada5efbc17ac08bb
Reviewed-on: https://go-review.googlesource.com/c/mod/+/366035
Trust: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

zip/zip_test.go

diff --git a/zip/zip_test.go b/zip/zip_test.go
index 810c07a..7fbebd5 100644
--- a/zip/zip_test.go
+++ b/zip/zip_test.go
@@ -999,7 +999,7 @@
 // we should not let that happen accidentally.
 func TestVCS(t *testing.T) {
 	if testing.Short() {
-		t.Skip()
+		t.Skip("skipping VCS cloning in -short mode")
 	}
 
 	var downloadErrorCount int32
@@ -1213,7 +1213,7 @@
 			if have, ok := haveVCS[test.vcs]; !ok {
 				t.Fatalf("unknown vcs: %s", test.vcs)
 			} else if !have {
-				t.Skip()
+				t.Skipf("no %s executable in path", test.vcs)
 			}
 			t.Parallel()
 
@@ -1723,10 +1723,11 @@
 // Note: some environments - and trybots - don't have git installed. This
 // function will cause the calling test to be skipped if that's the case.
 func gitInit(t *testing.T, dir string) {
-	t.Helper()
-
 	if _, err := exec.LookPath("git"); err != nil {
-		t.Skip("PATH does not contain git")
+		t.Skip("no git executable in path")
+	}
+	if runtime.GOOS == "plan9" {
+		t.Skip("plan9 git does not support the full git command line")
 	}
 
 	cmd := exec.Command("git", "init")
@@ -1752,10 +1753,11 @@
 }
 
 func gitCommit(t *testing.T, dir string) {
-	t.Helper()
-
 	if _, err := exec.LookPath("git"); err != nil {
-		t.Skip("PATH does not contain git")
+		t.Skip("no git executable in path")
+	}
+	if runtime.GOOS == "plan9" {
+		t.Skip("plan9 git does not support the full git command line")
 	}
 
 	cmd := exec.Command("git", "add", "-A")