Restore directory completions for subcommands

Fixes #6798

This re-adds some false positives: functions, builtins and abbreviations
are suggested after commands like sudo but I don't think anyone had
complained about that.

(cherry picked from commit 2a89873e6d686fcff1d26d0914a8b9f90b7cc308)

CHANGELOG.md

@@ -16,6 +16,7 @@ This is a bug fix release containing important fixes since 3.1.0. It
 - fixes 256 colors when $TERM starts with xterm, does not include "256color" and $TERM_PROGRAM is not set, as a workaround for old Terminal.app that was misapplied (#6701).
 - fixes error spew in the hg prompt when the working directory doesn't exist anymore (#6699)
 - fixes hosts with multiple aliases in the ssh config (#6698).
+- fixes subcommand tab completion for commands like "watch" where executables that are not in $PATH were not suggested (#6798).
 - improves compatibility with macOS code signing
 - improves the `flatpak` (#6581) and `kill` (#6636) completions
 - plus some typo and link fixes and minor adjustments to the documentation (#6627, #6642, #6646, #6647, #6657)

share/functions/__fish_complete_external_command.fish

@@ -1,3 +1,3 @@
 function __fish_complete_external_command
-    command find $PATH/ -maxdepth 1 -perm -u+x 2>&- | string match -r '[^/]*$'
+    complete -C "$argv[1]"
 end

share/functions/__fish_complete_subcommand.fish

@@ -49,7 +49,7 @@ function __fish_complete_subcommand -d "Complete subcommand" --no-scope-shadowin
     end
 
     if test $allow_functions_and_builtins = false && test (count $subcommand) -eq 1
-        __fish_complete_external_command
+        __fish_complete_external_command "$subcommand"
     else
         printf "%s\n" (complete -C "$subcommand")
     end