bootstrap oddity: .gitignore discrepancy

[Dave Kemper]

I haven't built a groff since 1.23.  When I did so today, the first
step produced a query I hadn't seen in a groff build before.

$ ./bootstrap
/bin/mv: overwrite '.gitignore'?

I said "y" to this, and the result was that the top-level .gitignore
file gained a "/build-aux" line at the top, before the first comment.
The line "/build-aux/" (with the trailing slash), already in the file
in the "artifacts expected in a clean tree" section, remained
unaltered, so "/build-aux" now appears twice, once with the trailing
slash and once without.

This has no effect I could discern on the rest of the bootstrap or build.