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.