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

libexec/pyenv

@@ -41,12 +41,15 @@ else
     # 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"
     )
   }
 fi

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() {

test/pyenv_ext.bats

@@ -28,3 +28,12 @@ load test_helper
   PYENV_FILE_ARG="${PYENV_TEST_DIR}/dir2/symlink.py" run pyenv echo PYENV_DIR
   assert_output "${PYENV_TEST_DIR}/dir1"
 }
+
+@test "should handle relative symlinks for file argument (#580)" {
+  mkdir -p "${PYENV_TEST_DIR}"
+  cd "${PYENV_TEST_DIR}"
+  touch file.py
+  ln -s file.py symlink.py
+  PYENV_FILE_ARG="symlink.py" run pyenv echo PYENV_DIR
+  assert_output "${PYENV_TEST_DIR}"
+}