varstash: add support for exported variables in subshells

tests/varstash_export.t

@@ -0,0 +1,58 @@
+Test varstash with exported variables in subshell.
+
+  $ source $TESTDIR/setup.sh || return 1
+
+Setup test environment.
+
+  $ mkdir sub
+  $ cd sub
+  $ echo 'echo ENTER; autostash MYVAR=changed; autostash MYEXPORT=changed_export' > $AUTOENV_FILE_ENTER
+  $ echo 'echo LEAVE; autounstash' > $AUTOENV_FILE_LEAVE
+
+Manually create auth file
+
+  $ test_autoenv_auth_env_files
+
+Set environment variable.
+
+  $ MYVAR=orig
+  $ export MYEXPORT=orig_export
+
+Activating the env stashes it and applies a new value.
+
+  $ cd .
+  ENTER
+  $ echo $MYVAR
+  changed
+  $ echo $MYEXPORT
+  changed_export
+
+The variable is not available in a subshell, only the exported one.
+
+  $ $SHELL -c 'echo ${MYVAR:-empty}; echo $MYEXPORT'
+  empty
+  changed_export
+
+Activate autoenv in the subshell.
+
+  $ $SHELL -c 'source $TEST_AUTOENV_PLUGIN_FILE; echo ${MYVAR}; echo $MYEXPORT'
+  ENTER
+  changed
+  changed_export
+
+"autounstash" should handle the exported variables.
+
+  $ $SHELL -c 'source $TEST_AUTOENV_PLUGIN_FILE; cd ..; echo ${MYVAR:-empty}; echo $MYEXPORT'
+  ENTER
+  LEAVE
+  empty
+  orig_export
+#
+# Exiting the subshell should restore.
+#
+#   $ pwd
+#   */varstash_export.t (glob)
+#   $ echo $MYVAR
+#   changed
+#   $ echo $MYEXPORT
+#   changed_export