gnulib-tool.py: Don't print Makefile.am edits that are already done. * pygnulib/GLMakefileTable.py (GLMakefileTable): Improve comments. (GLMakefileTable.__getitem__): Do not clone the result. * pygnulib/GLEmiter.py (GLEmiter.lib_Makefile_am, tests_Makefile_am): Avoid a KeyError when testing for 'var'. Use 'del' to remove a dictionary entry. * pygnulib/GLImport.py (GLImport.execute): Avoid a KeyError when testing for 'var'. Simplify loop over makefiletable.