mercurial: add .t files to .editorconfig

The notable entry here prevents the stripping of trailing whitespace,
which is important in .t tests.

Differential Revision: https://phab.mercurial-scm.org/D3702

.editorconfig

@@ -11,3 +11,8 b' trim_trailing_whitespace = true'
 indent_size = 8
 indent_style = tab
 trim_trailing_whitespace = true
+
+[*.t]
+indent_size = 2
+indent_style = space
+trim_trailing_whitespace = false