buildenv: using HEAD of a feed will not work

[pmelange]

One of the supported ways to use feeds.conf includes formatting in the following way:

src-git local_feed_name https://example.com/repo_name/something.git;branch_name

This, unfortunately, is not working. The resulting error is:

fatal: unable to access 'https://github.com/openwrt/packages.git;master/': The requested URL returned error: 400

see https://openwrt.org/docs/guide-developer/feeds#feed_configuration

[SvenRoederer]

This is also caused by the "glue-code" to create the modules-file from our feeds.conf, as written in #780 (comment).

PR #781 solves issue #780, but not sure if this will also handle the idea of the issue to use the HEAD of a repo.