bash: Refactor expand_path

Under Bash 3.2.57(1)-release and 4.4.12(1)-release on a MacBook Pro with
a 2.9GHz Intel Core i5 CPU and 8GB 1867MHz DDR3 RAM, this shaves off
O(0.25s) from the current test suite.

Before the change:

  46 tests, 0 failures

  real    0m3.851s
  user    0m2.273s
  sys     0m1.166s

After the change:

  real    0m3.595s
  user    0m2.171s
  sys     0m1.048s

libexec/bats

diff --git a/libexec/bats b/libexec/bats
index 5e90a9e..5cd95c6 100755
--- a/libexec/bats
+++ b/libexec/bats
@@ -56,11 +56,19 @@
 }
 
 expand_path() {
-  { cd "$(dirname "$1")" 2>/dev/null
-    local dirname="$PWD"
+  local path="${1%/}"
+  local dirname="${path%/*}"
+
+  if [[ "$dirname" == "$path" ]]; then
+    dirname="$PWD"
+  elif cd "$dirname" 2>/dev/null; then
+    dirname="$PWD"
     cd "$OLDPWD"
-    echo "$dirname/$(basename "$1")"
-  } || echo "$1"
+  else
+    printf '%s' "$path"
+    return
+  fi
+  printf -v "$2" '%s/%s' "$dirname" "${path##*/}"
 }
 
 abs_dirname "$0" 'BATS_LIBEXEC'
@@ -127,14 +135,16 @@
 
 filenames=()
 for filename in "${arguments[@]}"; do
+  expand_path "$filename" 'filename'
+
   if [ -d "$filename" ]; then
     shopt -s nullglob
-    for suite_filename in "$(expand_path "$filename")"/*.bats; do
+    for suite_filename in "$filename"/*.bats; do
       filenames["${#filenames[@]}"]="$suite_filename"
     done
     shopt -u nullglob
   else
-    filenames["${#filenames[@]}"]="$(expand_path "$filename")"
+    filenames["${#filenames[@]}"]="$filename"
   fi
 done