Closes gitaly#737

Now, when requesting a commit from the Repository model, the results are
not cached. This means we're fetching the same commit by oid multiple times
during the same request. To prevent us from doing this, we now cache
results. Caching is done only based on object id (aka SHA).

Given we cache on the Repository model, results are scoped to the
associated project, eventhough the change of two repositories having the
same oids for different commits is small.

This helps keep the abstraction layers simpler, and also keep the
interface of those methods consistent, in case of implementation
changes.

the rugged versions up to 0.26.0b3 had a bug concerning the signature
extraction. The extracted signature was not always the same, probably
due to a buffer (overflow) issue in libgit.

see https://github.com/libgit2/rugged/issues/608

Closes gitaly#396

This solves a problem where commits populated with Gitaly were not equal
to commits populated with Rugged. This is because Gitaly may not return
all fields of a commit for optimizations purposes, which resulted in
false negatives when comparing the same commit (commits with the same
sha) with different sources.