clipboard: close stdout when copying via xclip #11617

test_registers.vim can fail even if a clipboard manager is running.
If a clipboard manager is not running, this test always fails with xclip.
Use xsel as a workaround.
https://github.com/astrand/xclip/issues/20 suggests closing stdout
when sending input via stdin.

Environment
- Ubuntu Xenial
- Vim 7.4 (any app with broken clipboard code will do)
- Neovim nightly

Steps to reproduce:
0. Start the clipboard manager.
1. Open a file in Vim on Linux.
   Vim should have +clipboard enabled.
   'set clipboard='
2. Yank some text to the clipboard register.
3. Quit Vim.
4. Run 'cd /path/to/neovim/repo/'
5. Run 'make oldtest'.
   Do not run any individual tests.
   They likely pass with or without this fix.

Before fix: test_registers.vim can fail.
After fix: test_registers.vim always passes.

Close https://github.com/neovim/neovim/issues/7958

https://wiki.ubuntu.com/ClipboardPersistence#The_state_of_things

runtime/autoload/provider/clipboard.vim

@@ -172,6 +172,11 @@ function! s:clipboard.set(lines, regtype, reg) abort
   if jobid > 0
     call jobsend(jobid, a:lines)
     call jobclose(jobid, 'stdin')
+    " xclip does not close stdout,stderr when receiving input via stdin
+    if argv[0] ==# 'xclip'
+      call jobclose(jobid, 'stdout')
+      call jobclose(jobid, 'stderr')
+    endif
     let selection.owner = jobid
     let ret = 1
   else