dirname: fix another case

README.md

@@ -1067,10 +1067,9 @@ Alternative to the `dirname` command.
 dirname() {
     # Usage: dirname "path"
     dir=${1%%/}
+    [[ "${dir##*/*}" ]] && dir=.
     dir=${dir%/*}
 
-    [[ $1 == */* ]] || dir=.
-
     printf '%s\n' "${dir:-/}"
 }
 ```

manuscript/chapter5.txt

@@ -10,10 +10,9 @@ Alternative to the `dirname` command.
 dirname() {
     # Usage: dirname "path"
     dir=${1%%/}
+    [[ "${dir##*/*}" ]] && dir=.
     dir=${dir%/*}
 
-    [[ $1 == */* ]] || dir=.
-
     printf '%s\n' "${dir:-/}"
 }
 ```

test.sh

@@ -129,6 +129,9 @@ test_dirname() {
 
     result="$(dirname "/foo/foo")"
     assert_equals "$result" "/foo"
+
+    result="$(dirname "something/")"
+    assert_equals "$result" "."
 }
 
 test_basename() {