complete: fix completion of commands starting with -

When trying to complete a command starting with `-`, and more
specifically when trying to get the description of possible commands,
the dash was interpreted as an option for `__fish_describe_command`,
resulting in an "unknown option" most of the time.

This is a regression introduced when adding option parsing to
`__fish_describe_command`

Fixes 7fc27e9e5 (cygwin: improve handling of `.exe` file extension, 2025-11-22)
Fixes #12510

Closes #12522

src/complete.rs

@@ -999,11 +999,14 @@ fn complete_cmd_desc(&mut self, s: &wstr) {
         // systems with a large set of manuals, but it should be ok since apropos is only called once.
         // For Cygwin, also try to find the exact match for the non-exe name
         let lookup_cmd = sprintf!(
-            "functions -q __fish_describe_command &&{ __fish_describe_command %s %s}",
+            "functions -q __fish_describe_command &&{ __fish_describe_command -- %s %s}",
             &escape(cmd),
             &no_exe
                 .map(|(cmd_sans_exe, _)| {
-                    sprintf!("; __fish_describe_command --exact %s", escape(cmd_sans_exe))
+                    sprintf!(
+                        "; __fish_describe_command --exact -- %s",
+                        escape(cmd_sans_exe)
+                    )
                 })
                 .unwrap_or_default()[..]
         );

tests/checks/complete.fish

@@ -691,3 +691,12 @@ complete command-line-aware-completions -xa "(commandline --cursor; commandline
 complete -C"command-line-aware-completions "
 # CHECK: 31
 # CHECK: command-line-aware-completions
+
+begin
+    : >"$TMPDIR/-command-starting-with-dash"
+    chmod +x "$TMPDIR/-command-starting-with-dash"
+
+    set -l PATH "$TMPDIR" $PATH
+    complete -C"-command-starting-with"
+    # CHECK: -command-starting-with-dash{{\t}}command
+end