Add missing includes and forward declarations

I looped over the toplevel header files, creating a temporary two-line C
program for each consisting of
  #include "git-compat-util.h"
  #include $HEADER
This patch is the result of manually fixing errors in compiling those
tiny programs.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/revision.h b/revision.h
index c599c34..850e508 100644
--- a/revision.h
+++ b/revision.h
@@ -1,6 +1,7 @@
 #ifndef REVISION_H
 #define REVISION_H
 
+#include "commit.h"
 #include "parse-options.h"
 #include "grep.h"
 #include "notes.h"