guix gc: Add `--references' and `--referrers'.

* guix/scripts/gc.scm (show-help): Update.
  (%options): Add `--references' and `--referrers'.
  (guix-gc)[symlink-target, store-directory]: New procedures.
  Handle the `list-references' and `list-referrers' actions.
* tests/guix-gc.sh: Add tests for `--references'.
* doc/guix.texi (Invoking guix gc): Document `--references' and
  `--referrers'.

1 files changed, 12 insertions, 0 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index 6a9ebab1f6..ec784ce349 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -657,6 +657,18 @@ store---i.e., files and directories no longer reachable from any root.
 
 @item --list-live
 Show the list of live store files and directories.
+
+@end table
+
+In addition, the references among existing store files can be queried:
+
+@table @code
+
+@item --references
+@itemx --referrers
+List the references (respectively, the referrers) of store files given
+as arguments.
+
 @end table