gitcommit.cmake: stop needing TOPLEVEL_SOURCE_DIR.

It's always the same as the cwd when the script is invoked, and by
having the script get it _from_ its own cwd, we arrange a bit of
automatic normalisation in situations where you need to invoke it with
some non-canonical path like one ending in "/.." - which I'll do in
the next commit.

doc/CMakeLists.txt

@@ -21,7 +21,6 @@ if(HALIBUT AND PERL_EXECUTABLE)
       BYPRODUCTS ${INTERMEDIATE_VERSION_BUT}
       COMMAND ${CMAKE_COMMAND}
         -DGIT_EXECUTABLE=${GIT_EXECUTABLE}
-        -DTOPLEVEL_SOURCE_DIR=${CMAKE_SOURCE_DIR}
         -DOUTPUT_FILE=${INTERMEDIATE_VERSION_BUT}
         -DOUTPUT_TYPE=halibut
         -P ${CMAKE_SOURCE_DIR}/cmake/gitcommit.cmake