Fix warnings from 'make check'.

 - These files don't include 'builtin.h' causing sparse to complain that
   cmd_* isn't declared:

   builtin/clone.c:364, builtin/fetch-pack.c:797,
   builtin/fmt-merge-msg.c:34, builtin/hash-object.c:78,
   builtin/merge-index.c:69, builtin/merge-recursive.c:22
   builtin/merge-tree.c:341, builtin/mktag.c:156, builtin/notes.c:426
   builtin/notes.c:822, builtin/pack-redundant.c:596,
   builtin/pack-refs.c:10, builtin/patch-id.c:60, builtin/patch-id.c:149,
   builtin/remote.c:1512, builtin/remote-ext.c:240,
   builtin/remote-fd.c:53, builtin/reset.c:236, builtin/send-pack.c:384,
   builtin/unpack-file.c:25, builtin/var.c:75

 - These files have symbols which should be marked static since they're
   only file scope:

   submodule.c:12, diff.c:631, replace_object.c:92, submodule.c:13,
   submodule.c:14, trace.c:78, transport.c:195, transport-helper.c:79,
   unpack-trees.c:19, url.c:3, url.c:18, url.c:104, url.c:117, url.c:123,
   url.c:129, url.c:136, thread-utils.c:21, thread-utils.c:48

 - These files redeclare symbols to be different types:

   builtin/index-pack.c:210, parse-options.c:564, parse-options.c:571,
   usage.c:49, usage.c:58, usage.c:63, usage.c:72

 - These files use a literal integer 0 when they really should use a NULL
   pointer:

   daemon.c:663, fast-import.c:2942, imap-send.c:1072, notes-merge.c:362

While we're in the area, clean up some unused #includes in builtin files
(mostly exec_cmd.h).

Signed-off-by: Stephen Boyd <bebarino@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

This shrinks the top-level directory a bit, and makes it much more
pleasant to use auto-completion on the thing. Instead of

	[torvalds@nehalem git]$ em buil<tab>
	Display all 180 possibilities? (y or n)
	[torvalds@nehalem git]$ em builtin-sh
	builtin-shortlog.c     builtin-show-branch.c  builtin-show-ref.c
	builtin-shortlog.o     builtin-show-branch.o  builtin-show-ref.o
	[torvalds@nehalem git]$ em builtin-shor<tab>
	builtin-shortlog.c  builtin-shortlog.o
	[torvalds@nehalem git]$ em builtin-shortlog.c

you get

	[torvalds@nehalem git]$ em buil<tab>		[type]
	builtin/   builtin.h
	[torvalds@nehalem git]$ em builtin		[auto-completes to]
	[torvalds@nehalem git]$ em builtin/sh<tab>	[type]
	shortlog.c     shortlog.o     show-branch.c  show-branch.o  show-ref.c     show-ref.o
	[torvalds@nehalem git]$ em builtin/sho		[auto-completes to]
	[torvalds@nehalem git]$ em builtin/shor<tab>	[type]
	shortlog.c  shortlog.o
	[torvalds@nehalem git]$ em builtin/shortlog.c

which doesn't seem all that different, but not having that annoying
break in "Display all 180 possibilities?" is quite a relief.

NOTE! If you do this in a clean tree (no object files etc), or using an
editor that has auto-completion rules that ignores '*.o' files, you
won't see that annoying 'Display all 180 possibilities?' message - it
will just show the choices instead.  I think bash has some cut-off
around 100 choices or something.

So the reason I see this is that I'm using an odd editory, and thus
don't have the rules to cut down on auto-completion.  But you can
simulate that by using 'ls' instead, or something similar.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>