Allow cd . to re-enter the current directory

Closes #4543.

doc_src/cd.txt

@@ -16,6 +16,8 @@ Note that the shell will attempt to change directory without requiring `cd` if t
 
 Fish also ships a wrapper function around the builtin `cd` that understands `cd -` as changing to the previous directory. See also <a href="commands.html#prevd">`prevd`</a>. This wrapper function maintains a history of the 25 most recently visited directories in the `$dirprev` and `$dirnext` global variables. If you make those universal variables your `cd` history is shared among all fish instances.
 
+As a special case, `cd .` is equivalent to `cd $PWD`, which is useful in cases where a mountpoint has been recycled or a directory has been removed and recreated.
+
 \subsection cd-example Examples
 
 \fish

share/functions/cd.fish

@@ -27,6 +27,12 @@ function cd --description "Change directory"
         return $status
     end
 
+    # allow explicit "cd ." if the mount-point became stale in the meantime
+    if test "$argv" = "."
+        cd "$PWD"
+        return $status
+    end
+
     builtin cd $argv
     set -l cd_status $status