Fix incremental compilation

The macOS specific `package()` code changes the directory. This didn't cause any problems before 9135387cfa141e9ec27271bc41d5b6c43da90197 since `compile_commands.json` was written to the filesystem before `package()` was called. This is not the case anymore, which results in `compile_commands.json` being written into the `kitty.app` bundle. I fixed the problem by using a context manager to change the direcctory back after `kitty.app` was created.

setup.py

@@ -669,6 +669,18 @@ def compile_python(base_path):
         compileall.compile_dir(base_path, **kwargs)
 
 
+class ChDir(object):
+    def __init__(self, path):
+        self.old_dir = os.getcwd()
+        self.new_dir = path
+
+    def __enter__(self):
+        os.chdir(self.new_dir)
+
+    def __exit__(self, *args):
+        os.chdir(self.old_dir)
+
+
 def package(args, bundle_type):
     ddir = args.prefix
     if bundle_type == 'linux-freeze':
@@ -741,7 +753,7 @@ Categories=System;TerminalEmulator;
     if bundle_type.startswith('macos-'):  # macOS bundle gunk {{{
         import plistlib
         logo_dir = os.path.abspath(os.path.join('logo', appname + '.iconset'))
-        os.chdir(ddir)
+        with ChDir(ddir):
             os.mkdir('Contents')
             os.chdir('Contents')
             VERSION = '.'.join(map(str, version))