Improve required object computation for submodules

[andreaskurth]

Required objects are currently computed using git log, which stops at submodule boundaries.
Thus, for a repository that looks like

Memora.yml
some_dir/
submodule/
  bar/
  foo/

an artifact that depends on submodule/foo will miss in the cache also when submodule/bar changes. This leads to "false misses".

Instead, Memora should recognize submodule as submodule, descend into it, find the required commit for foo, and then find out which commit corresponds to this commit in the root repository. The last part might be the trickiest, because there is not necessarily a one-to-one correspondence between commits in the submodule and commits in the root repository.