patch 8.0.0749: some unicode digraphs are hard to remember

Problem:    Some unicode digraphs are hard to remember.
Solution:   Add alternatives with a backtick. (Chris Harding, closes #1861)
diff --git a/src/digraph.c b/src/digraph.c
index 6a57d8a..5e092c3 100644
--- a/src/digraph.c
+++ b/src/digraph.c
@@ -1876,6 +1876,13 @@
 	{'f', 'l', 0xfb02},
 	{'f', 't', 0xfb05},
 	{'s', 't', 0xfb06},
+
+	/* extra alternatives, easier to remember */
+	{'W', '`', 0x1e80},
+	{'w', '`', 0x1e81},
+	{'Y', '`', 0x1ef2},
+	{'y', '`', 0x1ef3},
+
 #      endif /* FEAT_MBYTE */
 
 	/* Vim 5.x compatible digraphs that don't conflict with the above */