vim: Import v8.0.1638

* 8.0.1638: popup test fails depending on environment variable
* 8.0.1637: no test for term_dumpdiff() options argument
* 8.0.1636: no test for term_dumpload() and term_dumpdiff()
* 8.0.1635: undefining _POSIX_THREADS causes problems with Python 3
* 8.0.1634: the ex_vimgrep() function is too long

gitlog.txt

@@ -1 +1 @@
-* [8.0.1633](https://github.com/vim/vim/releases/tag/v8.0.1633): a TextChanged autocmd triggers when it is defined\n* [8.0.1632](https://github.com/vim/vim/releases/tag/v8.0.1632): in a terminal dump NUL and space are different\n* [8.0.1631](https://github.com/vim/vim/releases/tag/v8.0.1631): testing with Vim running in terminal is a bit flaky
+* [8.0.1638](https://github.com/vim/vim/releases/tag/v8.0.1638): popup test fails depending on environment variable\n* [8.0.1637](https://github.com/vim/vim/releases/tag/v8.0.1637): no test for term_dumpdiff() options argument\n* [8.0.1636](https://github.com/vim/vim/releases/tag/v8.0.1636): no test for term_dumpload() and term_dumpdiff()\n* [8.0.1635](https://github.com/vim/vim/releases/tag/v8.0.1635): undefining _POSIX_THREADS causes problems with Python 3\n* [8.0.1634](https://github.com/vim/vim/releases/tag/v8.0.1634): the ex_vimgrep() function is too long