etc: Ensure guix.authenticate section is populated in post-merge hook.

This provides some extra automation.

* etc/git/post-merge [!guix.authenticate.introduction-commit]: Run
initial 'guix authenticate' command, as documented in (info "(guix) Building
from Git").

Fixes: <https://issues.guix.gnu.org/80564>
Change-Id: I5775514c6b8785213e2d0e834bdd7e3a961c8930
Reported-by: Tomas Volf <~@wolfsden.cz>

etc/git/post-merge

@@ -1,3 +1,9 @@
 #!/bin/sh
 # Authenticate the repo upon 'git pull' and similar.
+if ! git config guix.authentication.introduction-commit; then
+    exec guix git authenticate 9edb3f66fd807b096b48283debdcddccfea34bad \
+         "BBB0 2DDF 2CEA F6A8 0D1D  E643 A2A0 6DF2 A33A 54FA" \
+         -k origin/keyring
+else
     exec guix git authenticate
+fi