Add .isSubmodule

src/git-repository-async.js

@@ -288,4 +288,21 @@ module.exports = class GitRepositoryAsync {
 
     return this.projectAtRoot
   }
+
+  // Returns a {Promise} that resolves true if the given path is a submodule in
+  // the repository.
+  isSubmodule (_path) {
+    return this.repoPromise.then(function (repo) {
+      return repo.openIndex()
+    }).then(function (index) {
+      let entry = index.getByPath(_path)
+      let submoduleMode = 57344 // TODO compose this from libgit2 constants
+
+      if (entry.mode === submoduleMode) {
+        return true
+      } else {
+        return false
+      }
+    })
+  }
 }