__fish_print_help: Respect $MANPAGER

Fixes #9488

CHANGELOG.rst

@@ -25,6 +25,7 @@ Interactive improvements
 - The history pager now uses more screen space, usually half the screen (:issue:`9458`)
 - Variables that were set while the locale was C (i.e. ASCII) will now properly be encoded if the locale is switched (:issue:`2613`, :issue:`9473`).
 - Escape during history search restores the original commandline again (regressed in 3.6.0).
+- Using ``--help`` on builtins now respects the $MANPAGER variable in preference to $PAGER (:issue:`9488`).
 
 New or improved bindings
 ^^^^^^^^^^^^^^^^^^^^^^^^

share/functions/__fish_print_help.fish

@@ -126,8 +126,12 @@ function __fish_print_help --description "Print help message for the specified f
     end | string replace -ra '^       ' '' |
         begin
             set -l pager less
+            # Try both PAGER and MANPAGER, last one wins
             set -q PAGER
-            and echo $PAGER | read -at pager
+            and echo -- $PAGER | read -at pager
+            set -q MANPAGER
+            and echo -- $MANPAGER | read -at pager
+
             not isatty stdout
             and set pager cat # cannot use a builtin here
             # similar to man, but add -F to quit paging when the help output is brief (#6227)