treewide: add direct includes currently only pulled in transitively

The next commit will remove a bunch of unnecessary includes, but to do
so, we need some of the lower level direct includes that files rely on
to be explicitly specified.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/commit-graph.c

@@ -11,6 +11,7 @@
 #include "object-store-ll.h"
 #include "progress.h"
 #include "replace-object.h"
+#include "strbuf.h"
 #include "tag.h"
 #include "trace2.h"