functions/man: don't export MANPATH unnecessarily

We always try to prepend our man path to $MANPATH before calling man.
But this is not necessary when we're gonna display the output of
"status get-file man/man1/abbr.1".

Overriding man pages can cause issues like
https://github.com/fish-shell/fish-shell/issues/11472
https://gitlab.com/man-db/man-db/-/merge_requests/15

which by themselves are maybe not enough reason to avoid exporting
MANPATH, but given that embedded man pages might become the only
choice in future (see #12037), we should reduce the scope of our
custom MANPATH.

share/functions/man.fish

@@ -6,38 +6,35 @@ if not command -qs man
 end
 
 function man
-    # Work around the "builtin" manpage that everything symlinks to,
-    # by prepending our fish datadir to man. This also ensures that man gives fish's
-    # man pages priority, without having to put fish's bin directories first in $PATH.
-
-    # Preserve the existing MANPATH, and default to the system path (the empty string).
     set -l manpath
-    if set -q MANPATH
-        set manpath $MANPATH
-    else if set -l p (command man -p 2>/dev/null)
-        # NetBSD's man uses "-p" to print the path.
-        # FreeBSD's man also has a "-p" option, but that requires an argument.
-        # Other mans (men?) don't seem to have it.
-        #
-        # Unfortunately NetBSD prints things like "/usr/share/man/man1",
-        # while not allowing them as $MANPATH components.
-        # What it needs is just "/usr/share/man".
-        #
-        # So we strip the last component.
-        # This leaves a few wrong directories, but that should be harmless.
-        set manpath (string replace -r '[^/]+$' '' $p)
-    else
-        set manpath ''
-    end
-    # Notice the shadowing local exported copy of the variable.
-    set -lx MANPATH $manpath
-
-    # Prepend fish's man directory if available.
     if not __fish_is_standalone
-        set -l fish_manpath $__fish_data_dir/man
-        if test -d $fish_manpath
-            set MANPATH $fish_manpath $MANPATH
-        end
+        and set -l fish_manpath (path filter -d $__fish_data_dir/man)
+        # Prepend fish's man directory if available.
+
+        # Work around the "builtin" manpage that everything symlinks to,
+        # by prepending our fish datadir to man. This also ensures that man gives fish's
+        # man pages priority, without having to put fish's bin directories first in $PATH.
+
+        # Preserve the existing MANPATH, and default to the system path (the empty string).
+        set manpath $fish_manpath (
+            if set -q MANPATH
+                string join -- \n $MANPATH
+            else if set -l p (command man -p 2>/dev/null)
+                # NetBSD's man uses "-p" to print the path.
+                # FreeBSD's man also has a "-p" option, but that requires an argument.
+                # Other mans (men?) don't seem to have it.
+                #
+                # Unfortunately NetBSD prints things like "/usr/share/man/man1",
+                # while not allowing them as $MANPATH components.
+                # What it needs is just "/usr/share/man".
+                #
+                # So we strip the last component.
+                # This leaves a few wrong directories, but that should be harmless.
+                string replace -r '[^/]+$' '' $p
+            else
+                echo ''
+            end
+        )
     end
 
     if test (count $argv) -eq 1
@@ -54,6 +51,9 @@ function man
             (string escape --style=regex -- $basename) __fish_man
         __fish_with_status functions --erase __fish_man
     else
+        set -q manpath[1]
+        and set -lx MANPATH $manpath
+
         command man $argv
     end
 end