Fix single mount rename panic

Fixes #12141

hugolib/filesystems/basefs_test.go

@@ -234,6 +234,7 @@ foo
 	b := hugolib.Test(t, files)
 	bfs := b.H.BaseFs
 	watchFilenames := bfs.WatchFilenames()
+	//   []string{"/hugo_stats.json", "/content", "/content2", "/themes/t1/layouts", "/themes/t1/layouts/_default", "/themes/t1/static"}
 	b.Assert(watchFilenames, qt.HasLen, 6)
 }
 
@@ -543,6 +544,30 @@ files/f2.txt false
 `)
 }
 
+func TestMountIssue12141(t *testing.T) {
+	files := `
+-- hugo.toml --
+disableKinds = ["taxonomy", "term"]
+[module]
+[[module.mounts]]
+source = "myfiles"
+target = "static"
+[[module.mounts]]
+source = "myfiles/f1.txt"
+target = "static/f2.txt"
+-- myfiles/f1.txt --
+f1
+`
+	b := hugolib.Test(t, files)
+	fs := b.H.BaseFs.StaticFs("")
+
+	b.AssertFs(fs, `
+. true
+f1.txt false
+f2.txt false
+`)
+}
+
 func checkFileCount(fs afero.Fs, dirname string, c *qt.C, expected int) {
 	c.Helper()
 	count, names, err := countFilesAndGetFilenames(fs, dirname)