gcf2client: split out request API from regular git
While Gcf2Client is designed to mimic what git-cat-file writes
to stdout, its request format is different to support requests
with a git repository path included.
We'll highlight the distinction and make the GitAsyncCat support
code easier-to-follow as a result.
Since Gcf2Client relies on DS, we can rely on DS-specific code
here, too, and use a single Unix socket instead of separate
input and output pipes, reducing memory overhead in both users
and kernel space. Due to the interactive nature of requests and
responses, the buffer size limitations of Unix sockets on Linux
seems inconsequential here (just like it is for existing "git
cat-file --batch" use).