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/pretty.h b/pretty.h
index 5c85d94..7359d31 100644
--- a/pretty.h
+++ b/pretty.h
@@ -1,7 +1,11 @@
 #ifndef PRETTY_H
 #define PRETTY_H
 
+#include "cache.h"
+#include "string-list.h"
+
 struct commit;
+struct strbuf;
 
 /* Commit formats */
 enum cmit_fmt {