Isolate shared HTTP request functionality

Move shared HTTP request functionality out of http-fetch and http-push,
and replace the two fwrite_buffer/fwrite_buffer_dynamic functions with
one fwrite_buffer function that does dynamic buffering.  Use slot
callbacks to process responses to fetch object transfer requests and
push transfer requests, and put all of http-push into an #ifdef check
for curl multi support.

Signed-off-by: Nick Hengeveld <nickh@reactrix.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Makefile

@@ -387,7 +387,8 @@ $(SIMPLE_PROGRAMS) : git-%$X : %.o
 	$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) $(filter %.o,$^) \
 		$(LIB_FILE) $(SIMPLE_LIB)
 
-git-http-fetch$X: fetch.o
+git-http-fetch$X: fetch.o http.o
+git-http-push$X: http.o
 git-local-fetch$X: fetch.o
 git-ssh-fetch$X: rsh.o fetch.o
 git-ssh-upload$X: rsh.o