Autocomplete installed dists in `pip show ...`. (#4749)

* Autocomplete installed dists in `pip show ...`.

* 🎨 Assign to a variable to satisfy linters

news/4749.feature

+The command-line autocompletion engine ``pip show`` now autocompletes installed
+distribution names.

src/pip/_internal/__init__.py

@@ -87,8 +87,12 @@ def autocomplete():
         # special case: 'help' subcommand has no options
         if subcommand_name == 'help':
             sys.exit(1)
-        # special case: list locally installed dists for uninstall command
-        if subcommand_name == 'uninstall' and not current.startswith('-'):
+        # special case: list locally installed dists for show and uninstall
+        should_list_installed = (
+            subcommand_name in ['show', 'uninstall'] and
+            not current.startswith('-')
+        )
+        if should_list_installed:
             installed = []
             lc = current.lower()
             for dist in get_installed_distributions(local_only=True):