This will let us to maximize the capability of our asynchronous
git API. This lets us avoid relying on EOF to notify lei2mail
workers; thus giving us the option of running fewer lei_xsearch
worker processes in parallel than local sources.
I tried using a synchronous git API; and even with libgit2 in
the same process to avoid the IPC cost failed to match the
throughput afforded by this change. This is because libgit2 is
built (at least on Debian) with the SHA-1 collision code enabled
and ubc_check stuff was dominating my profiles.