Fix diff parsing.

If we pass a `LibGit2Sharp.Patch` to `DiffUtilities.ParseFragment` it includes a header that should not be counted when calculating `DiffLine`. Make the parser ignore diff content before the first diff chunk header.

Author: grokys

src/GitHub.Exports/Models/DiffUtilities.cs

@@ -16,7 +16,7 @@ public static IEnumerable<DiffChunk> ParseFragment(string diff)
             {
                 string line;
                 DiffChunk chunk = null;
-                int diffLine = 0;
+                int diffLine = -1;
                 int oldLine = -1;
                 int newLine = -1;
 
@@ -31,6 +31,8 @@ public static IEnumerable<DiffChunk> ParseFragment(string diff)
                             yield return chunk;
                         }
 
+                        if (diffLine == -1) diffLine = 0;
+
                         chunk = new DiffChunk
                         {
                             OldLineNumber = oldLine = int.Parse(headerMatch.Groups[1].Value),
@@ -70,7 +72,7 @@ public static IEnumerable<DiffChunk> ParseFragment(string diff)
                         }
                     }
 
-                    ++diffLine;
+                    if (diffLine != -1) ++diffLine;
                 }
 
                 if (chunk != null)

test/GitHub.InlineReviews.UnitTests/Services/DiffServiceTests.cs

@@ -41,17 +41,17 @@ public void ShouldParsePr960()
                 // -    public class UsageTracker : IUsageTracker
                 Assert.Equal(17, result[0].Lines[7].OldLineNumber);
                 Assert.Equal(-1, result[0].Lines[7].NewLineNumber);
-                Assert.Equal(12, result[0].Lines[7].DiffLineNumber);
+                Assert.Equal(8, result[0].Lines[7].DiffLineNumber);
 
                 // +    public sealed class UsageTracker : IUsageTracker, IDisposable
                 Assert.Equal(-1, result[0].Lines[8].OldLineNumber);
                 Assert.Equal(18, result[0].Lines[8].NewLineNumber);
-                Assert.Equal(13, result[0].Lines[8].DiffLineNumber);
+                Assert.Equal(9, result[0].Lines[8].DiffLineNumber);
 
                 //      IConnectionManager connectionManager;
                 Assert.Equal(26, result[0].Lines[17].OldLineNumber);
                 Assert.Equal(25, result[0].Lines[17].NewLineNumber);
-                Assert.Equal(22, result[0].Lines[17].DiffLineNumber);
+                Assert.Equal(18, result[0].Lines[17].DiffLineNumber);
             }
         }
     }