Fix abs_dirname for relative symlinks in same directory

Ref (abs_dirname in bats): https://github.com/sstephenson/bats/pull/224
Ref: https://github.com/rbenv/rbenv/pull/868

Fixes https://github.com/pyenv/pyenv/issues/580

plugins/python-build/bin/python-build

@@ -60,20 +60,21 @@ resolve_link() {
 }
 
 abs_dirname() {
-  local cwd="$(pwd)"
   local path="$1"
 
-  # Use a subshell to avoid modifying the current path
+  # Use a subshell to avoid changing the current path
   (
   while [ -n "$path" ]; do
-    cd "${path%/*}"
-    local name="${path##*/}"
+    cd_path="${path%/*}"
+    if [[ "$cd_path" != "$path" ]]; then
+      cd "$cd_path"
+    fi
+    name="${path##*/}"
     path="$(resolve_link "$name" || true)"
   done
 
-  pwd
+  echo "$PWD"
   )
-# cd "$cwd"
 }
 
 capitalize() {