* guix/scripts/build.scm (show-help): Document --manifest argument.
(options->things-to-build): When given a manifest, evaluate all the entries.
* tests/guix-build.sh: Add test for --manifest.
* doc/guix.texi (Additional Build Options): Mention --manifest.
* etc/completion/bash/guix: Complete file name if 'guix build' argument is
-m.
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