bats: Refactor resolve_link, abs_dirname

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(0.4s) from the current test suite.

Before the change:

  46 tests, 0 failures

  real    0m4.392s
  user    0m2.489s
  sys     0m1.467s

After the change:

  real    0m3.980s
  user    0m2.312s
  sys     0m1.233s

libexec/bats

@@ -26,18 +26,29 @@ help() {
   echo
 }
 
+BATS_READLINK=
+
 resolve_link() {
-  $(type -p greadlink readlink | head -1) "$1"
+  if [[ -z "$BATS_READLINK" ]]; then
+    if command -v 'greadlink' >/dev/null; then
+      BATS_READLINK='greadlink'
+    elif command -v 'readlink' >/dev/null; then
+      BATS_READLINK='readlink'
+    else
+      BATS_READLINK='true'
+    fi
+  fi
+  "$BATS_READLINK" "$1" || return 0
 }
 
 abs_dirname() {
-  local cwd="$(pwd)"
+  local cwd="$PWD"
   local path="$1"
 
   while [ -n "$path" ]; do
     cd "${path%/*}"
     local name="${path##*/}"
-    path="$(resolve_link "$name" || true)"
+    path="$(resolve_link "$name")"
   done
 
   pwd