Fix github language statistics.

As documented in https://github.com/github/linguist#overrides and from the
conversation in github-linguist/linguist#3540 we add a
.gitattributes file to the project wherein we pretend that the sql files are
all vendored-in.

This should allow GitHub to realize that pgloader is all about Common Lisp
and not a single line of PLpgSQL...

Author: dimitri

.gitattributes

@@ -0,0 +1 @@
+test/**/*.sql linguist-vendored