maint: re-anchor all .gitignore entries.

maint: re-anchor all .gitignore entries.

* bootstrap: Copy from gnulib's latest for the fix to
automatically anchor entries it constructs.
* gnulib: Update to latest just so it has the same bootstrap.
* .gitignore, build-aux/.gitignore, doc/.gitignore:
* lib/.gitignore, m4/.gitignore, po/.gitignore:
* runtime-po/.gitignore: Re-anchor all entries.
(cherry picked from commit e503b9cbbebde5e0ecccdcc864969b9a609b940d)

Conflicts:

    build-aux/.gitignore
    doc/.gitignore
    src/.gitignore