Add ability to configure diff tool by env variable

docs/mdsource/diff-tool.source.md

@@ -68,6 +68,21 @@ include: diffTools
 snippet: DisableDiff
 
 
+## Diff Tool order
+
+
+### Default
+
+include: defaultDiffToolOrder
+
+
+### Custom order
+
+Set an `Verify.DiffToolOrder` with the preferred order of diff tool resolution. The value can be comma (`,`), pipe (`|`), or space separated.
+
+For example `VisualStudio,Meld` will result in VisualStudio then Meld then all other tools being the order.
+
+
 ## DiffEngine
 
 **API SUBJECT TO CHNAGE IN MINOR RELEASES**

src/DiffEngine.Tests/DiffToolsTest.BadEnvironmentVariable.verified.txt

@@ -0,0 +1 @@
+Unable to parse tool from `Verify.DiffToolOrder` environment variable: Foo

src/DiffEngine.Tests/DiffToolsTest.ParseEnvironmentVariable.verified.txt

@@ -0,0 +1,4 @@
+[
+  'VisualStudio',
+  'Meld'
+]

src/DiffEngine.Tests/DiffToolsTest.cs

@@ -1,6 +1,8 @@
-using System.Diagnostics;
+using System;
+using System.Diagnostics;
 using System.IO;
 using System.Linq;
+using System.Threading.Tasks;
 using DiffEngine;
 using VerifyXunit;
 using Xunit;
@@ -18,6 +20,19 @@ public class DiffToolsTest :
     //    Assert.True(DiffTools.IsDetectedFor(DiffTool.BeyondCompare, "png"));
     //}
 
+    [Fact]
+    public Task ParseEnvironmentVariable()
+    {
+        return Verify(DiffTools.ParseEnvironmentVariable("VisualStudio,Meld"));
+    }
+
+    [Fact]
+    public Task BadEnvironmentVariable()
+    {
+        var exception = Assert.Throws<Exception>(() => DiffTools.ParseEnvironmentVariable("Foo").ToList());
+        return Verify(exception.Message);
+    }
+
     [Fact]
     public void WriteFoundTools()
     {

src/DiffEngine.Tests/defaultDiffToolOrder.include.md

@@ -0,0 +1,17 @@
+ * BeyondCompare
+ * P4Merge
+ * AraxisMerge
+ * Meld
+ * SublimeMerge
+ * Kaleidoscope
+ * CodeCompare
+ * WinMerge
+ * DiffMerge
+ * TortoiseMerge
+ * TortoiseGitMerge
+ * TortoiseIDiff
+ * KDiff3
+ * TkDiff
+ * VisualStudioCode
+ * VisualStudio
+ * Rider