gps: Insulate against .git as file

`git ls-remote` was being run without a current working directory, which
- for some versions of git - has the unfortunate side effect of still
looking for a local .git in the cwd hierarchy, and failing if that .git
is a file instead of a directory (as it might be if the user is working
beneath a git submodule).

This relocates the call to somewhere safer, under gps' control, so that
we've a better (though not foolproof) guarantee of not colliding with a
.git file.

Fixes #1338.

gps/source_test.go

@@ -10,6 +10,7 @@ import (
 	"io/ioutil"
 	"log"
 	"os"
+	"path/filepath"
 	"reflect"
 	"testing"
 
@@ -36,6 +37,7 @@ func testSourceGateway(t *testing.T) {
 		os.RemoveAll(cachedir)
 		cancelFunc()
 	}()
+	os.Mkdir(filepath.Join(cachedir, "sources"), 0777)
 
 	do := func(wantstate sourceState) func(t *testing.T) {
 		return func(t *testing.T) {

gps/vcs_source.go

@@ -242,6 +242,16 @@ func (s *gitSource) listVersions(ctx context.Context) (vlist []PairedVersion, er
 	r := s.repo
 
 	cmd := commandContext(ctx, "git", "ls-remote", r.Remote())
+	// We want to invoke from a place where it's not possible for there to be a
+	// .git file instead of a .git directory, as git ls-remote will choke on the
+	// former and erroneously quit. However, we can't be sure that the repo
+	// exists on disk yet at this point; if it doesn't, then instead use the
+	// parent of the local path, as that's still likely a good bet.
+	if r.CheckLocal() {
+		cmd.SetDir(r.LocalPath())
+	} else {
+		cmd.SetDir(filepath.Dir(r.LocalPath()))
+	}
 	// Ensure no prompting for PWs
 	cmd.SetEnv(append([]string{"GIT_ASKPASS=", "GIT_TERMINAL_PROMPT=0"}, os.Environ()...))
 	out, err := cmd.CombinedOutput()

gps/vcs_source_test.go

@@ -53,6 +53,7 @@ func testGitSourceInteractions(t *testing.T) {
 			t.Errorf("removeAll failed: %s", err)
 		}
 	}()
+	os.Mkdir(filepath.Join(cpath, "sources"), 0777)
 
 	n := "github.com/sdboyer/gpkt"
 	un := "https://" + n
@@ -149,6 +150,7 @@ func testGopkginSourceInteractions(t *testing.T) {
 			t.Errorf("removeAll failed: %s", err)
 		}
 	}()
+	os.Mkdir(filepath.Join(cpath, "sources"), 0777)
 
 	tfunc := func(opath, n string, major uint64, evl []Version) {
 		un := "https://" + opath
@@ -533,6 +535,8 @@ func TestGitSourceListVersionsNoHEAD(t *testing.T) {
 	defer h.Cleanup()
 	h.TempDir("smcache")
 	cpath := h.Path("smcache")
+	os.Mkdir(filepath.Join(cpath, "sources"), 0777)
+
 	h.TempDir("repo")
 	repoPath := h.Path("repo")
 
@@ -599,6 +603,7 @@ func TestGitSourceListVersionsNoDupes(t *testing.T) {
 			t.Errorf("removeAll failed: %s", err)
 		}
 	}()
+	os.Mkdir(filepath.Join(cpath, "sources"), 0777)
 
 	n := "github.com/carolynvs/deptest-importers"
 	un := "https://" + n