fix(ignore, fs): allow loading ignore patterns behind symlink (fixes #10785) (#10786)

The recent change to disallow following symlinks by default conflicts
with the expected behavior of .stignore. This adds a new flag which may
be passed to OpenFile to skip default symlink-forbidding open flag.

---------

Signed-off-by: Jakob Borg <jakob@kastelo.net>

lib/ignore/ignore_test.go

@@ -1731,3 +1731,37 @@ func testEscape(t *testing.T, tests []escapeTest, noErrors bool) {
 		}
 	}
 }
+
+// TestIgnoreThroughSymlink verifies that an ignore file can be loaded when
+// it is itself a symlink pointing at the real file. This exercises the
+// SkipDefaultOpenFlags path, as our normal Open enforces O_NOFOLLOW. We use
+// a real filesystem here because the fake one does not implement the
+// O_NOFOLLOW semantics that make this distinction meaningful.
+func TestIgnoreThroughSymlink(t *testing.T) {
+	if build.IsWindows {
+		t.Skip("symlinks not supported on Windows")
+	}
+
+	testFS := fs.NewFilesystem(fs.FilesystemTypeBasic, t.TempDir())
+
+	// The real ignore file lives under a different name, and .stignore is a
+	// symlink pointing at it.
+	if err := fs.WriteFile(testFS, "real-ignores", []byte("bfile\n"), 0o666); err != nil {
+		t.Fatal(err)
+	}
+	if err := testFS.CreateSymlink("real-ignores", ".stignore"); err != nil {
+		t.Fatal(err)
+	}
+
+	pats := New(testFS, WithCache(true))
+	if err := pats.Load(".stignore"); err != nil {
+		t.Fatal(err)
+	}
+
+	if !pats.Match("bfile").IsIgnored() {
+		t.Error("bfile should be ignored")
+	}
+	if pats.Match("afile").IsIgnored() {
+		t.Error("afile should not be ignored")
+	}
+}