Add length check for log indent level

Add length check for log indent level to avoid exceptions like this:

Error Message:
 System.ArgumentOutOfRangeException : Length cannot be less than zero.
Parameter name: length
Stack Trace:
  at System.String.Substring (System.Int32 startIndex, System.Int32 length) [0x0004a] in <f3f2aa82c3a04d48845485ce37124803>:0
  at GitVersion.Logger+<>c__DisplayClass19_0.<IndentLog>b__0 () [0x00010] in <05fc21618d444a47be7b45d99fa1d3c6>:0
  at GitVersion.Logger+ActionDisposable.Dispose () [0x00000] in <05fc21618d444a47be7b45d99fa1d3c6>:0
  at GitVersion.VersionCalculation.BaseVersionCalculator.GetBaseVersion (GitVersion.GitVersionContext context) [0x00211] in
<05fc21618d444a47be7b45d99fa1d3c6>:0
  at GitVersion.VersionCalculation.NextVersionCalculator.FindVersion (GitVersion.GitVersionContext context) [0x0003f] in
<05fc21618d444a47be7b45d99fa1d3c6>:0
  at GitVersion.GitVersionFinder.FindVersion (GitVersion.GitVersionContext context) [0x000a1] in <05fc21618d444a47be7b45d99fa1d3c6>:0
  at GitVersionCore.Tests.GitToolsTestingExtensions.ExecuteGitVersion (GitVersion.GitVersionContext context) [0x00007] in
<e141d799e05748d0bf9753d21cdcadd7>:0
  at GitVersionCore.Tests.GitToolsTestingExtensions.GetVersion (GitTools.Testing.RepositoryFixtureBase fixture, GitVersion.Config configuration,
LibGit2Sharp.IRepository repository, System.String commitId, System.Boolean isForTrackedBranchOnly, System.String targetBranch) [0x00030] in
<e141d799e05748d0bf9753d21cdcadd7>:0
  at GitVersionCore.Tests.GitToolsTestingExtensions.AssertFullSemver (GitTools.Testing.RepositoryFixtureBase fixture, GitVersion.Config
configuration, System.String fullSemver, LibGit2Sharp.IRepository repository, System.String commitId, System.Boolean isForTrackedBranchOnly,
System.String targetBranch) [0x00050] in <e141d799e05748d0bf9753d21cdcadd7>:0
  at GitVersionCore.Tests.GitToolsTestingExtensions.AssertFullSemver (GitTools.Testing.RepositoryFixtureBase fixture, System.String fullSemver,
LibGit2Sharp.IRepository repository, System.String commitId, System.Boolean isForTrackedBranchOnly, System.String targetBranch) [0x00007] in
<e141d799e05748d0bf9753d21cdcadd7>:0
  at DocumentationSamples.GitFlowFeatureBranch () [0x000a2] in <e141d799e05748d0bf9753d21cdcadd7>:0
  at (wrapper managed-to-native) System.Reflection.MonoMethod.InternalInvoke(System.Reflection.MonoMethod,object,object[],System.Exception&)
  at System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder,
System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00032] in <f3f2aa82c3a04d48845485ce37124803>:0

https://dev.azure.com/GitTools/GitVersion/_build/results?buildId=191&view=logs&jobId=a5e52b91-c83f-5429-4a68-c246fc63a4f7&taskId=d6b35a2b-3a7d-590e-76eb-a415813199f6&lineStart=225&lineEnd=226&colStart=1&colEnd=1

Author: asbjornu

src/GitVersionCore/Logger.cs

@@ -26,7 +26,8 @@ public static IDisposable IndentLog(string operationDescription)
             indent = indent + "  ";
             return new ActionDisposable(() =>
             {
-                indent = indent.Substring(0, indent.Length - 2);
+                var length = indent.Length - 2;
+                indent = length > 0 ? indent.Substring(0, length) : indent;
                 WriteInfo(string.Format(CultureInfo.InvariantCulture, "End: {0} (Took: {1:N}ms)", operationDescription, DateTime.Now.Subtract(start).TotalMilliseconds));
             });
         }