Add 'guix graph'.
* guix/scripts/graph.scm, tests/graph.scm, tests/guix-graph.sh,
doc/images/coreutils-bag-graph.dot, doc/images/coreutils-graph.dot: New
files.
* Makefile.am (MODULES): Add guix/scripts/graph.scm.
(SH_TESTS): Add tests/guix-graph.sh.
(SCM_TESTS): Add tests/graph.scm.
* doc.am (DOT_FILES, DOT_VECTOR_GRAPHICS): New variables.
(EXTRA_DIST): Use them.
(dist_infoimage_DATA): Use $(DOT_FILES).
(pdf-local, info-local, ps-local): Likewise.
* doc/guix.texi (Packages with Multiple Outputs): Add cross-reference to 'guix
graph'.
(Invoking guix gc): Likewise.
(Invoking guix graph): New section.
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