Modularize commit-walker

This turns the extern functions to be provided by the backend into a
struct of pointers, renames the functions to be more
namespace-friendly, and updates http-fetch to this interface. It
removes the unused include from http-push.c. It makes git-http-fetch a
builtin (with the implementation a separate file, accessible
directly).

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/git.c b/git.c
index 56ae8cc..41fe73e 100644
--- a/git.c
+++ b/git.c
@@ -344,6 +344,9 @@
 		{ "get-tar-commit-id", cmd_get_tar_commit_id },
 		{ "grep", cmd_grep, RUN_SETUP | USE_PAGER },
 		{ "help", cmd_help },
+#ifndef NO_CURL
+		{ "http-fetch", cmd_http_fetch, RUN_SETUP },
+#endif
 		{ "init", cmd_init_db },
 		{ "init-db", cmd_init_db },
 		{ "log", cmd_log, RUN_SETUP | USE_PAGER },