Do not fail in GetCommitsInfo with no .gitmodules

This fixes GetCommitsInfo returning

 GetSubModule (/somemodule): object does not exist [id: , rel_path: .gitmodules]

when /somemodule is a ref entry but no .gitmodules file is present. Instead
failing we just check for IsErrNotExist and return empty submodule url.

This also fixes HTTP 500 internal error in Gogs for case described above.

Author: nanoant

tree_entry.go

@@ -144,7 +144,7 @@ func (tes Entries) GetCommitsInfo(commit *Commit, treePath string) ([][]interfac
 		go func(i int) {
 			cinfo := commitInfo{entryName: tes[i].Name()}
 			sm, err := commit.GetSubModule(path.Join(treePath, tes[i].Name()))
-			if err != nil {
+			if err != nil && !IsErrNotExist(err) {
 				cinfo.err = fmt.Errorf("GetSubModule (%s/%s): %v", treePath, tes[i].Name(), err)
 				revChan <- cinfo
 				return