gitignore: use only one .gitignore file, in the top-level directory * doc/.gitignore, doc/amhello/.gitignore, lib/Automake/.gitignore, lib/Automake/tests/.gitignore, tests/.gitignore: Deleted, their contents merged into ... * .gitignore: ... this top-level file. Since we are at it, improve it by anchoring files where it makes sense.