Also prepend "./" for filter if a filename starts with "-"

This is now added to the two commands that definitely deal with
relative paths.

It doesn't work for e.g. `path basename`, because after removing the
dirname prepending a "./" doesn't refer to the same file, and the
basename is also expected to not contain any slashes.

doc_src/cmds/path.rst

@@ -170,6 +170,8 @@ The filter options can either be given as multiple options, or comma-separated -
 
 With ``--invert``, the meaning of the filtering is inverted - any path that wouldn't pass (including by not existing) passes, and any path that would pass fails.
 
+When a path starts with ``-``, ``path filter`` will prepend ``./`` to avoid it being interpreted as an option otherwise.
+
 It returns 0 if at least one path passed the filter.
 
 ``path is`` is shorthand for ``path filter -q``, i.e. just checking without producing output, see :ref:`The is subcommand <cmd-path-is>`.

src/builtins/path.cpp

@@ -815,7 +815,15 @@ static int path_filter(parser_t &parser, io_streams_t &streams, int argc, const
                 if (ok == opts.invert) continue;
             }
 
-            path_out(streams, opts, *arg);
+            // We *know* this is a filename,
+            // and so if it starts with a `-` we *know* it is relative
+            // to $PWD. So we can add `./`.
+            if (!arg->empty() && arg->front() == L'-') {
+                wcstring out = L"./" + *arg;
+                path_out(streams, opts, out);
+            } else {
+                path_out(streams, opts, *arg);
+            }
             n_transformed++;
             if (opts.quiet) return STATUS_CMD_OK;
         }

tests/checks/path.fish

@@ -120,6 +120,11 @@ path normalize -- -/foo -foo/foo
 path normalize -- ../-foo
 # CHECK: ../-foo
 
+# This goes for filter as well
+touch -- -foo
+path filter -f -- -foo
+# CHECK: ./-foo
+
 # We need to remove the rest of the path because we have no idea what its value looks like.
 path resolve bin//sh | string match -r -- 'bin/bash$'
 # The "//" is squashed, and the symlink is resolved.