gps: vcs: suppress git password prompts

[jmank88]

What does this do / why do we need it?

This change suppresses git password prompts in more places. Cherry picked from #1250 (f1f0bac).

Which issue(s) does this PR fix?

It might fix: #1356

[jmank88]

tip $GOCACHE CI failures:

testproj.go:112: go standard error:
testproj.go:113: initializing cache in $GOCACHE: mkdir /test-dep-home-does-not-exist: permission denied

[carolynvs]

When dep runs git commands, does it inherit our current environment variables? I'm wondering if we can just set the environment variable once, instead of directly on the git command?

[jmank88]

I'm not sure - I copied from this:

dep/gps/vcs_source.go

Lines 241 to 246 in 80e94e8

	func (s *gitSource) listVersions(ctx context.Context) (vlist []PairedVersion, err error) {
	r := s.repo
	cmd := commandContext(ctx, "git", "ls-remote", r.Remote())
	// Ensure no prompting for PWs
	cmd.SetEnv(append([]string{"GIT_ASKPASS=", "GIT_TERMINAL_PROMPT=0"}, os.Environ()...))

[carolynvs]

Ah gotcha, yeah it doesn't inherit any environment variables then. 👍

[jmank88]

This now includes a bonus commit to resolve some intermittent appveyor failures.

[carolynvs]

If you fix our AppVeyor builds I owe you a lb of chocolates and your beverage of choice! 👍