Add special '$GID' variable in Eshell

See bug#64529.

* lisp/eshell/esh-var.el (eshell-variable-aliases-list): Add '$GID'.

* test/lisp/eshell/esh-var-tests.el (esh-var-test/gid-var): New test.

* doc/misc/eshell.texi (Variables): Document '$GID'.

* etc/NEWS: Announce this change (and the previous change for '$UID').

doc/misc/eshell.texi

@@ -993,6 +993,13 @@ variable is connection-aware, so when the current directory is remote,
 its value will be @acronym{UID} for the user associated with that
 remote connection.
 
+@vindex $GID
+@item $GID
+This returns the effective @acronym{GID} for the current user.  Like
+@code{$UID}, this variable is connection-aware, so when the current
+directory is remote, its value will be @acronym{GID} for the user
+associated with that remote connection.
+
 @vindex $_
 @item $_
 This refers to the last argument of the last command.  With a

etc/NEWS

@@ -202,6 +202,11 @@ or get a sublist of elements 2 through 4 with '$my-list[2..5]'.  For
 more information, see the "(eshell) Dollars Expansion" node in the
 Eshell manual.
 
++++
+*** Eshell's '$UID' and '$GID' variables are now connection-aware.
+Now, when expanding '$UID' or '$GID' in a remote directory, the value
+is the user or group ID associated with the remote connection.
+
 ---
 *** Eshell now uses 'field' properties in its output.
 In particular, this means that pressing the '<home>' key moves the

lisp/eshell/esh-var.el

@@ -163,6 +163,7 @@ if they are quoted with a backslash."
     ("LINES" ,(lambda () (window-body-height nil 'remap)) t t)
     ("INSIDE_EMACS" eshell-inside-emacs t)
     ("UID" ,(lambda () (file-user-uid)) nil t)
+    ("GID" ,(lambda () (file-group-gid)) nil t)
 
     ;; for esh-ext.el
     ("PATH" (,(lambda () (string-join (eshell-get-path t) (path-separator)))

test/lisp/eshell/esh-var-tests.el

@@ -829,6 +829,10 @@ it, since the setter is nil."
   "Test that $UID is equivalent to (user-uid) for local directories."
   (eshell-command-result-equal "echo $UID" (user-uid)))
 
+(ert-deftest esh-var-test/gid-var ()
+  "Test that $GID is equivalent to (group-gid) for local directories."
+  (eshell-command-result-equal "echo $GID" (group-gid)))
+
 (ert-deftest esh-var-test/last-status-var-lisp-command ()
   "Test using the \"last exit status\" ($?) variable with a Lisp command."
   (with-temp-eshell