Fix regression causing no submodule info

This fixes regression introduced in ebd9fb2253,
that GetSubModules was always returning empty submodule info because
c.submoduleCache != nil check was always true since parseCommitData was setting
commit.submoduleCache = newObjectCache() so it was always not-nil.

Now we initialize submoduleCache in first GetSubModules call, next call will
return cached entries.

commit.go

@@ -195,6 +195,7 @@ func (c *Commit) GetSubModules() (*objectCache, error) {
 	}
 
 	scanner := bufio.NewScanner(rd)
+	c.submoduleCache = newObjectCache()
 	var ismodule bool
 	var path string
 	for scanner.Scan() {