Added ./u entries in .gitignore

This adds two entries in the .gitignore file:

    ./u
    ./u/*

for ignoring such a subdirectory. The gentle user is now able to put
her/his utility scripts in there.

For example, one could put an astyle helper (callastyle.sh) in there:

    #!/bin/bash
    astyle --options=./etc/astyle

as typing the astyle call by hand can be annoying.

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I want this patch to go into the codebase, as I can for example put vim-ex snippets in this directory and call them from vim, so they can do work for me!