zip: fix on Windows

Zip paths are slash-delimited, so using filepath with them is
incorrect.

Change-Id: I5ad1ee90dea97427b9b291b6fbc52e5bdaf0b678
Reviewed-on: https://go-review.googlesource.com/c/mod/+/474815
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Heschi Kreinick <heschi@google.com>
Auto-Submit: Heschi Kreinick <heschi@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>

zip/zip.go

@@ -663,7 +663,7 @@ func filesInGitRepo(dir, rev, subdir string) ([]File, error) {
 		if n == "" {
 			continue
 		}
-		n = strings.TrimPrefix(n, string(filepath.Separator))
+		n = strings.TrimPrefix(n, "/")
 
 		fs = append(fs, zipFile{
 			name: n,

zip/zip_test.go

@@ -1532,7 +1532,7 @@ c/`)))
 			}
 			wantMap := map[string]bool{}
 			for _, f := range tc.wantFiles {
-				p := filepath.Join("example.com", "foo", "bar@v0.0.1", f)
+				p := path.Join("example.com", "foo", "bar@v0.0.1", f)
 				wantMap[p] = true
 			}