Customize the log file for the check targets based on the input file.

The idea being that it's better to segregate feedback, just in case
someone is working on several translations.  Now the check log will
appear in `./build/src/nvim/po/check-${LANG}.log`.

src/nvim/po/check.vim

@@ -6,7 +6,8 @@
 
 if 1	" Only execute this if the eval feature is available.
 
-redir! > check.log
+let filename = "check-" . expand("%:t:r") . ".log"
+exe 'redir! > ' . filename
 
 " Function to get a split line at the cursor.
 " Used for both msgid and msgstr lines.