Always #include corresponding .h in .c files

While doing this, remove declarations from header files where the
corresponding definition is declared "static" in order to avoid build
errors.

Also re-order existing headers in ui-*.c so that the file-specific
header always comes immediately after "cgit.h", helping with future
consistency.

Signed-off-by: John Keeping <john@keeping.me.uk>

ui-diff.h

@@ -3,9 +3,6 @@
 
 extern void cgit_print_diff_ctrls();
 
-extern void cgit_print_diffstat(const unsigned char *old_sha1,
-				const unsigned char *new_sha1);
-
 extern void cgit_print_diff(const char *new_hex, const char *old_hex,
 			    const char *prefix, int show_ctrls);