gitweb: fix generation of "gitweb.js"

gitweb: fix generation of "gitweb.js"

In 19d8fe7da65 (Makefile: extract script to generate gitweb.js,
2024-12-06) we have extracted the logic to build "gitweb.js" into a
separate script. As part of that the rules that builds the script
has gained a new dependency on that script.

This refactoring is broken though because we use "$^" to determine
the set of JavaScript files that need to be concatenated, and this
implicit variable now also contains the build script itself. As a
result, the build script ends up ni the generated "gitweb.js" file,
which is wrong.

Fix the issue by filtering out non-JavaScript files.

Based-on-patch-by: Thorsten Glaser <tg@debian.org>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>