git authenticate: Document ‘--end’.

* guix/scripts/git/authenticate.scm (show-help): Document ‘--end’.
* doc/guix.texi (Invoking guix git authenticate): Likewise.

Reported-by: Tomas Volf <~@wolfsden.cz>
Change-Id: Ia646203ce2f721487de547c76b9488754c70db66

doc/guix.texi

@@ -7632,6 +7632,9 @@ contain OpenPGP public keys in @file{.key} files, either in binary form
 or ``ASCII-armored''.  By default the keyring is loaded from the branch
 named @code{keyring}.
 
+@item --end=@var{commit}
+Authenticate revisions up to @var{commit}.
+
 @item --stats
 Display commit signing statistics upon completion.
 

guix/scripts/git/authenticate.scm

@@ -100,6 +100,8 @@ Authenticate the given Git checkout using COMMIT/SIGNER as its introduction.\n")
   (display (G_ "
   -k, --keyring=REFERENCE
                          load keyring from REFERENCE, a Git branch"))
+  (display (G_ "
+      --end=COMMIT       authenticate revisions up to COMMIT"))
   (display (G_ "
       --stats            display commit signing statistics upon completion"))
   (display (G_ "