zip: set PWD consistently for commands in subdirectories

Some platforms (notably macOS) use symlinks for /tmp, so if we fail to
set PWD appropriately, any resulting errors may refer to the wrong
directory, making those errors more confusing.

Change-Id: Ibc899d7aca138d9e016171e34e7e4aab4895a4df
Reviewed-on: https://go-review.googlesource.com/c/mod/+/375914
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

zip/zip.go

@@ -641,6 +641,7 @@ func filesInGitRepo(dir, rev, subdir string) ([]File, error) {
 		cmd.Args = append(cmd.Args, subdir)
 	}
 	cmd.Dir = dir
+	cmd.Env = append(os.Environ(), "PWD="+dir)
 	cmd.Stdout = &stdout
 	cmd.Stderr = &stderr
 	if err := cmd.Run(); err != nil {
@@ -679,11 +680,12 @@ func isGitRepo(dir string) bool {
 	stdout := &bytes.Buffer{}
 	cmd := exec.Command("git", "rev-parse", "--git-dir")
 	cmd.Dir = dir
+	cmd.Env = append(os.Environ(), "PWD="+dir)
 	cmd.Stdout = stdout
 	if err := cmd.Run(); err != nil {
 		return false
 	}
-	gitDir := strings.TrimSpace(string(stdout.Bytes()))
+	gitDir := strings.TrimSpace(stdout.String())
 	if !filepath.IsAbs(gitDir) {
 		gitDir = filepath.Join(dir, gitDir)
 	}