RST reader: more accurate parsing of references.

Previously we erroneously included the enclosing
backticks in a reference ID (closes #4156).

This change also disables interpretation of
syntax inside references, as in docutils.
So, there is no emphasis in

    `my *link*`_

1 files changed, 10 insertions, 0 deletions

diff --git a/test/command/4156.md b/test/command/4156.md
new file mode 100644
index 000000000..073537d1f
--- /dev/null
+++ b/test/command/4156.md
@@ -0,0 +1,10 @@
+```
+% pandoc -f rst
+.. _`SOMEID`:
+
+foo
+^D
+<div id="SOMEID">
+<p>foo</p>
+</div>
+```