Localized git manpages fail to expand {build_dir} includes

[yangzhg]

URL for broken page

https://git-scm.com/docs/git/zh_HANS-CN

Problem

The localized git manpages show warnings like:

Missing `zh_HANS-CN/{build_dir}/cmds-mainporcelain.adoc`

I also checked es, fr, pt_BR, ru, sv, and uk; they have the same problem.

The files are present in jnavila/git-html-l10n. It looks like
expand_l10n() matches the {build_dir}/ prefix but only removes
include:: from the matched line, so it tries to resolve a literal path
such as:

zh_HANS-CN/{build_dir}/cmds-mainporcelain.adoc

The English expand_content() path already handles this case.

Operating system and browser

Not browser-specific. The warning is in the generated HTML.

Steps to reproduce

1. Open any localized git manpage listed above.
2. Scroll to the command lists.
3. The included command lists are replaced by missing-file warnings.

Other details

Related English fixes: #1942 and #1993.

The translated pages will need a forced rebuild after this is fixed.