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() {

repo_commit.go

@@ -36,7 +36,6 @@ func (repo *Repository) GetTagCommitID(name string) (string, error) {
 // \n\n separate headers from message
 func parseCommitData(data []byte) (*Commit, error) {
 	commit := new(Commit)
-	commit.submoduleCache = newObjectCache()
 	commit.parents = make([]sha1, 0, 1)
 	// we now have the contents of the commit object. Let's investigate...
 	nextline := 0