USE_CURL_MULTI and http.h in general

The changes I made to the commit-walker fetch code turn out not to work 
with USE_CURL_MULTI, and I'm not at all clear on how to deal with it. I 
think the current code for USE_CURL_MULTI also can't work for having 
multiple things in the same executable using http.h, which will be a 
problem for having both ls-remote and http-fetch as part of builtin-fetch, 
as well as eventually calling http-push without an exec from builtin-push. 
Is there anyone around who understands it?

	-Daniel
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