catacomb-import-update: --convert: Check the .mk is unchanged

catacomb-import-update: --convert: Check the .mk is unchanged

The .mk lists donor and output files, the relationship between which
is primarily recorded in catacomb-import-update, but also encoded in
the .mk which is loaded by the makefiles to know what to build and
when.  This change to --convert detects if the .mk corresponding to
this version of the script is different to that which generated the
actually-effective one in the import commit.

If this check trips, --import must be rerun.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>