Make include_top comments only apply to the next statement

Previously adding an include_top comment would make it apply
for the rest of the file. This was confusing behavior, and
could also cause extra roots to be searched in a situation
like the following:

\#RBC# include_top foo
include $(VAR)/file.mk
\#RBC# include_top bar
include $(VAR)/file.mk

Here the second include would have file.mk from both the foo
and bar directories in its _entry dictionary.

Bug: 213508006
Test: go test
Change-Id: If3430594759bee1390255400fe29b43d77f7b6a6