Preserve top level comments on update of imports (#55)

Fixes #54

main.go

@@ -325,6 +325,10 @@ func (r *patchRunner) Apply(filename string, f *ast.File) (*ast.File, bool) {
 func cleanupFilePos(tfile *token.File, cl engine.Changelog, comments []*ast.CommentGroup) {
 	linesToDelete := make(map[int]struct{})
 	for _, dr := range cl.ChangedIntervals() {
+		if dr.Start == token.NoPos {
+			continue
+		}
+
 		for i := tfile.Line(dr.Start); i < tfile.Line(dr.End); i++ {
 			if i > 0 {
 				linesToDelete[i] = struct{}{}

testdata/replace_import_with_top_level_comment

@@ -0,0 +1,32 @@
+-- in.patch --
+@@
+var randomIdentifier identifier
+@@
+-import "cmd/internal/edit"
++import "github.com/foo/bar/internal/edit"
+
+edit.randomIdentifier
+
+-- comment.in.go --
+// Copyright 2022 ...
+
+package main
+
+import (
+	"cmd/internal/edit"
+)
+
+func main() {
+	_ = edit.Buffer
+}
+
+-- comment.out.go --
+// Copyright 2022 ...
+
+package main
+
+import "github.com/foo/bar/internal/edit"
+
+func main() {
+	_ = edit.Buffer
+}