[CI Visibility] Fix detached head situation (#6594)

## Summary of changes
In a detached head situation, create a synthetic branch name like
`auto:git-detached-head`.

## Reason for change
An empty branch name (as when a detached head happens) leads to many of
the TestVis features to be disabled.

## Implementation details

## Test coverage

## Other details
<!-- Fixes #{issue} -->

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Author: daniel-romano-DD

tracer/src/Datadog.Trace/Ci/IntelligentTestRunnerClient.cs

@@ -88,8 +88,9 @@ public IntelligentTestRunnerClient(string? workingDirectory, CIVisibilitySetting
         _customConfigurations = GetCustomTestsConfigurations(_settings.TracerSettings.GlobalTags);
 
         _repositoryUrl = GetRepositoryUrl();
-        _branchName = GetBranchName();
         _commitSha = GetCommitSha();
+        _branchName = GetBranchName();
+
         _apiRequestFactory = CIVisibility.GetRequestFactory(_settings.TracerSettings, TimeSpan.FromSeconds(45));
 
         const string settingsUrlPath = "api/v2/libraries/tests/services/setting";
@@ -1269,7 +1270,15 @@ private string GetBranchName()
         }
 
         var gitOutput = GitCommandHelper.RunGitCommand(_workingDirectory, "branch --show-current", MetricTags.CIVisibilityCommands.GetBranch);
-        return gitOutput?.Output.Replace("\n", string.Empty) ?? string.Empty;
+        var res = gitOutput?.Output.Replace("\n", string.Empty) ?? string.Empty;
+
+        if (string.IsNullOrEmpty(res))
+        {
+            Log.Warning("ITR: empty branch indicates a detached head at commit {Commit}", _commitSha);
+            res = $"auto:git-detached-head";
+        }
+
+        return res;
     }
 
     private string GetCommitSha()

tracer/test/Datadog.Trace.ClrProfiler.IntegrationTests/CI/TestingFrameworkImpactedTests.cs

@@ -29,7 +29,7 @@ public abstract class TestingFrameworkImpactedTests : TestingFrameworkTest
 #pragma warning disable SA1401 // FieldsMustBePrivate
         protected const string ModifiedLine = "// Modified by TestingFrameworkImpactedTests.cs";
         protected const int ExpectedTestCount = 16;
-        protected const string GitHubBaseSha = "a700b56e12ddcbad5d19a2fa8852a15518ab205b";
+        protected string baseSha = string.Empty;
         protected string repositoryRoot = string.Empty;
         protected string repo = string.Empty;
         protected string branch = string.Empty;
@@ -76,7 +76,7 @@ protected string GetSettingsJson(bool enabled = false)
 
         protected string GetDiffFilesJson(bool baseCommit = true)
         {
-            var commitValue = baseCommit ? GitHubBaseSha : string.Empty;
+            var commitValue = baseCommit ? baseSha : string.Empty;
             return $$"""
             {
               "data": {
@@ -167,7 +167,6 @@ protected override Dictionary<string, string> DefineCIEnvironmentValues(Dictiona
         {
             // Base sets Azure CI values. Take those we can reuse for Git Hub
             repo = values[CIEnvironmentValues.Constants.AzureBuildRepositoryUri];
-            branch = values[CIEnvironmentValues.Constants.AzureBuildSourceBranch];
 
             return values;
         }
@@ -198,13 +197,13 @@ protected void InjectGitHubActionsSession(bool setupPr = true, bool? enabled = t
                 SetEnvironmentVariable(ConfigurationKeys.CIVisibility.ImpactedTestsDetectionEnabled, enabled.Value ? "True" : "False");
             }
 
-            static string GetEventJsonFile()
+            string GetEventJsonFile()
             {
                 string content = $$"""
                 {
                   "pull_request": {
                     "base": {
-                      "sha": "{{GitHubBaseSha}}"
+                      "sha": "{{baseSha}}"
                     }
                   }
                 }
@@ -243,6 +242,26 @@ private void InitGit()
                 output = RunGitCommand("branch --show-current");
             }
 
+            if (output.ExitCode == 0)
+            {
+                // Retrieve branch name
+                branch = output.Output.Trim();
+            }
+
+            if (output.ExitCode == 0)
+            {
+                // Retrieve last commit
+                output = RunGitCommand("rev-parse --verify HEAD");
+                if (output.ExitCode == 0)
+                {
+                    baseSha = output.Output.Trim();
+                    if (string.IsNullOrEmpty(branch))
+                    {
+                        branch = $"auto:git-detached-head";
+                    }
+                }
+            }
+
             if (output.ExitCode == 0)
             {
                 // Retrieve WS root directory
@@ -251,7 +270,7 @@ private void InitGit()
                 {
                     gitAvailable = true;
                     repositoryRoot = output.Output.Trim();
-                    Output.WriteLine($"Git available. Repository: {repositoryRoot}");
+                    Output.WriteLine($"Git available. Repository: {repositoryRoot} Branch: {branch} Sha: {baseSha}");
                 }
             }