gitignore: Add leading slashes where appropriate
    
    Going through the list of gitignore patterns without a leading slash,
    this adds a leading slash where it appears to have been forgotten.
    
    Some gitignore patterns like ".deps/" or "Makefile" clearly should
    match everywhere, so those definitively need no leading slash.
    
    For some patterns like "ascii.bitmaps", it is unclear where in the
    source tree they should match. Those patterns are kept as they are,
    matching the patterns in the whole tree of subdirectories.
    
    Signed-off-by: Hans Ulrich Niedermann <hun@n-dimensional.de>
    Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>