Support backtracking when there is a failed match

The prevents gaps in matched lines being accepted.

Author: jcansdale

src/GitHub.Exports/Models/DiffUtilities.cs

@@ -82,24 +82,29 @@ public static IEnumerable<DiffChunk> ParseFragment(string diff)
 
         public static DiffLine Match(IEnumerable<DiffChunk> diff, IList<DiffLine> target)
         {
-            int j = 0;
-
             if (target.Count == 0)
             {
                 return null; // no lines to match
             }
 
             foreach (var source in diff)
             {
+                var matches = 0;
                 for (var i = source.Lines.Count - 1; i >= 0; --i)
                 {
-                    if (source.Lines[i].Content == target[j].Content)
+                    if (source.Lines[i].Content == target[matches].Content)
                     {
-                        if (++j == target.Count || i == 0)
+                        matches++;
+                        if (matches == target.Count || i == 0)
                         {
-                            return source.Lines[i + j - 1];
+                            return source.Lines[i + matches - 1];
                         }
                     }
+                    else
+                    {
+                        i += matches;
+                        matches = 0;
+                    }
                 }
             }
 

test/GitHub.InlineReviews.UnitTests/Models/DiffUtilitiesTests.cs

@@ -203,6 +203,7 @@ public class TheMatchMethod
             [InlineData("+a.+x.+x.", "+a.+x.+x.", 2)]
             [InlineData("+a.+x.+x.+b.+x.+x.", "+a.+x.", 1)]
             [InlineData("+a.+x.+x.+b.+x.+x.", "+b.+x.", 4)]
+            [InlineData("+a.+b.+x", "+a.+x.", -1)] // backtrack when there is a failed match
             public void MatchLine(string lines1, string lines2, int skip /* -1 for no match */)
             {
                 var header = "@@ -1 +1 @@";