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

CHANGELOG.md

@@ -18,6 +18,7 @@ IMPROVEMENTS:
 * Reduce network access by trusting local source information and only pulling from upstream when necessary ([#1250](https://github.com/golang/dep/pull/1250)).
 * Update our dependency on Masterminds/semver to follow upstream again now that [Masterminds/semver#67](https://github.com/Masterminds/semver/pull/67) is merged([#1792](https://github.com/golang/dep/pull/1792)).
 * Don't exclude `Godeps` folder ([#1822](https://github.com/golang/dep/issues/1822)).
+* Export the content of git submodules to the vendor directory ([#1909](https://github.com/golang/dep/pull/1909)).
 
 WIP:
 * Enable importing external configuration from dependencies during init (#1277). This is feature flagged and disabled by default.

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
 }