gps: export content of git submodules

The "git checkout-index" command is unaware of git submodules,
therefore their content was missed when exporting files.

With "git submodule foreach" the same operation (read-tree +
checkout-index) can also be done for all submodules, recursively. The
downside is that "foreach" invokes a shell command with certain
variables set. To achieve the desired functionality, we have to rely
on POSIX shell support.

Author: pohly

gps/vcs_source.go

@@ -203,6 +203,18 @@ func (s *gitSource) exportRevisionTo(ctx context.Context, rev Revision, to strin
 		}
 	}
 
+	// Now do the same for any submodule, using a shell command that "git submodule"
+	// iterates over.
+	{
+		cmd := commandContext(ctx, "git", "submodule", "foreach", "--recursive",
+			"cd \"$toplevel/.git/modules/$name\" && mv index origindex && trap 'mv origindex index' EXIT && git read-tree \"$sha1\" && git checkout-index -a --prefix=\""+to+"/$path/\"",
+		)
+		cmd.SetDir(r.LocalPath())
+		if out, err := cmd.CombinedOutput(); err != nil {
+			return errors.Wrap(err, string(out))
+		}
+	}
+
 	return nil
 }