Summary:
Update deserializeGitBlob() to work on an IOBuf, rather than an rvalue
reference to a string.
The ugliness about having to wrap a std::string in a managed IOBuf is now
hidden inside the StoreResult class, rather than being something that the
GitBlob code has to know about.
Reviewed By: bolinfest
Differential Revision: D3403977
fbshipit-source-id: 0c58c019557050d6e201c1a462fa051c2526674a