Don't use FileNotFoundException for non-IO operation

Fixed `if (`.

Author: jcansdale

src/GitHub.App/Services/PullRequestService.cs

@@ -309,7 +309,7 @@ public IObservable<Tuple<string, string>> ExtractDiffFiles(
                 string mergeBase = await gitClient.GetPullRequestMergeBase(repo, remote.Name, baseSha, headSha, baseRef, pullRequest.Number);
                 if (mergeBase == null)
                 {
-                    throw new FileNotFoundException($"Couldn't find merge base between {baseSha} and {headSha}.");
+                    throw new InvalidOperationException($"Couldn't find merge base between {baseSha} and {headSha}.");
                 }
 
                 string left;
@@ -349,7 +349,7 @@ static bool HasPreamble(string file, Encoding encoding)
             {
                 foreach (var b in encoding.GetPreamble())
                 {
-                    if(b != stream.ReadByte())
+                    if (b != stream.ReadByte())
                     {
                         return false;
                     }

src/UnitTests/GitHub.App/Services/PullRequestServiceTests.cs

@@ -68,7 +68,7 @@ public async void MergeBaseNotAvailable_ThrowsFileNotFoundException()
             var checkedOut = false;
             var expectMessage = $"Couldn't find merge base between {baseSha} and {headSha}.";
 
-            var ex = await Assert.ThrowsAsync<FileNotFoundException>(() => ExtractDiffFiles(baseSha, baseFileContent, headSha, headFileContent, mergeBaseSha, mergeBaseFileContent,
+            var ex = await Assert.ThrowsAsync<InvalidOperationException>(() => ExtractDiffFiles(baseSha, baseFileContent, headSha, headFileContent, mergeBaseSha, mergeBaseFileContent,
                                 fileName, checkedOut));
             Assert.Equal(expectMessage, ex.Message);
         }