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.