gitignore: more use of anchors

gitignore: more use of anchors

* .gitignore: Anchor files that are intended to be ignored only
if found in the same directory of the `.gitignore' file, not also
in its subdirectories.
* doc/.gitignore, doc/amhello/.gitignore, lib/Automake/.gitignore,
lib/Automake/tests/.gitignore, tests/.gitignore: Likewise.  Also,
where needed, add new entries that were once implied by the
non-anchored entries in the upper-level `.gitignore' files.