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/fs/filesystem.go

@@ -12,6 +12,7 @@ import (
 	"fmt"
 	"io"
 	"io/fs"
+	"math/bits"
 	"os"
 	"path/filepath"
 	"strings"
@@ -173,8 +174,18 @@ const (
 	OptSync       = os.O_SYNC
 	OptTruncate   = os.O_TRUNC
 	OptWriteOnly  = os.O_WRONLY
+	OptFollow     = 1 << (bits.UintSize - 2)
 )
 
+func init() {
+	// Ensure OptFollow doesn't clash with any other flag.
+	flags := OptAppend | OptCreate | OptExclusive | OptReadOnly | OptReadWrite | OptSync | OptTruncate | OptWriteOnly
+	flags &= OptFollow
+	if flags != 0 {
+		panic("bug: flag clash")
+	}
+}
+
 // SkipDir is used as a return value from WalkFuncs to indicate that
 // the directory named in the call is to be skipped. It is not returned
 // as an error by any function.