Fix "Edit This File" path (#790)

* Acquire the branch name as a new variable, keeping github.ref_name as a fallback.

Author: cotti

.github/workflows/preview-build.yml

@@ -115,6 +115,7 @@ jobs:
         env:
           PR_NUMBER: ${{ github.event.pull_request.number }}
           GITHUB_REF_NAME: ${{ github.ref_name }}
+          GITHUB_PR_REF_NAME: ${{ github.event.pull_request.head.ref }}
         run: |
           case "${GITHUB_EVENT_NAME}" in
             "merge_group" | "pull_request" | "pull_request_target")

src/Elastic.Markdown/IO/Discovery/GitCheckoutInformation.cs

@@ -67,7 +67,7 @@ public static GitCheckoutInformation Create(IDirectoryInfo? source, IFileSystem
 			branch = branch.Replace("ref: ", string.Empty);
 		}
 		else
-			branch = "detached/head";
+			branch = Environment.GetEnvironmentVariable("GITHUB_PR_REF_NAME") ?? Environment.GetEnvironmentVariable("GITHUB_REF_NAME") ?? "detached/head";
 
 		var ini = new IniFile();
 		using var stream = gitConfig.OpenRead();