cmd/go/internal/modfetch/gitrepo: failing on Plan 9

[0intro]

CL 118095 added gitrepo tests. These tests are failing on Plan 9 (when git is available in the path), since they expect a full-featured Git command, while on Plan 9, the git tool has been emulated as a simple rc script.

ok  	cmd/go/internal/modfetch	0.108s
2018/06/16 22:52:00 git clone --mirror https://vcs-test.golang.org/git/gitrepo1 /tmp/gitrepo-test-603600095/gitrepo2 in : exit status: 'git 742985: fatal':
	fatal: website 'vcs-test.golang.org' isn't supported
FAIL	cmd/go/internal/modfetch/gitrepo	0.251s

See https://build.golang.org/log/89774241214d5243596cbcd6b771f7cb55eccb9d

[rsc]

Feel free to send a CL to skip the tests on Plan 9.
We're not likely to simplify our git usage.

[gopherbot]

Change https://golang.org/cl/122455 mentions this issue: cmd/go: skip gitrepo tests on Plan 9