When launching child process set the PWD environment variable

This allows shells to display the symlink path to a directory instead
of the resolved path, when the specified directory is a symlink. Fixes #1595

docs/changelog.rst

@@ -129,6 +129,9 @@ To update |kitty|, :doc:`follow the instructions <binary>`.
   It can now increment font size and reset it. It also only acts on the
   active top-level window, by default (:iss:`1581`)
 
+- When launching child processes set the :code:`PWD` environment variable
+  (:iss:`1595`)
+
 0.13.3 [2019-01-19]
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