Treat . as punctuation

Closes #388

CHANGELOG.md

@@ -1,5 +1,10 @@
 ## 0.37 (unreleased)
 
+### Diffing
+
+Standalone `.` is now considered punctuation in the diff cost model,
+which improves diff results in some cases.
+
 ### Build
 
 Fixed an issue with building on Windows with gcc.

src/diff/graph.rs

@@ -414,7 +414,7 @@ fn allocate_if_new<'syn, 'b>(
 /// This check is deliberately conservative, becuase it's hard to
 /// accurately recognise punctuation in a language-agnostic way.
 fn looks_like_punctuation(content: &str) -> bool {
-    content == "," || content == ";"
+    content == "," || content == ";" || content == "."
 }
 
 /// Compute the neighbours of `v` if we haven't previously done so,