Update cache when no specific revision is set for dependency

[jm4R]

When CPM_SOURCE_CACHE is set and GIT_TAG is set to specific revision (eg. commit hash) the CPM doesn't have to update the dependency as we are sure that no changes could be made for specific commit.

On the other hand, when CPM_SOURCE_CACHE is set and GIT_TAG is set to branch name (eg. develop), the CPM should first fetch the dependency for changes on that branch.