Optimize retrieval and propagation of X-Sha

Simplify retrieval by leveraging JQ support in GH CLI, and always ensure we propagate the latest value for the X-Sha header.

src/File/GitHub.cs

@@ -33,6 +33,9 @@ public static class GitHub
         public static bool TryIsInstalled(out string output)
             => Process.TryExecute("gh", "--version", out output) && output.StartsWith("gh version");
 
+        public static bool TryApi(string endpoint, string jq, out string? output)
+            => Process.TryExecute("gh", $"api {endpoint} --jq \"{jq}\"", out output);
+
         public static bool TryApi(string endpoint, out JToken? json)
         {
             json = null;

src/File/Http/GitHubRawHandler.cs

@@ -59,14 +59,10 @@ protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage
                 (originalEtag != newEtag || originalSha == null) &&
                 parts.Length > 2 &&
                 GitHub.IsInstalled &&
-                GitHub.TryApi($"repos/{parts[0]}/{parts[1]}/commits?per_page=1&path={string.Join('/', parts.Skip(4))}", out var json) &&
-                json is JArray commits &&
-                commits[0] is JObject obj &&
-                obj.Property("sha") is JProperty prop &&
-                prop != null &&
-                prop.Value.Type == JTokenType.String)
+                GitHub.TryApi($"repos/{parts[0]}/{parts[1]}/commits?per_page=1&path={string.Join('/', parts.Skip(4))}", ".[0]?.sha", out var json) &&
+                json is { Length: > 0 })
             {
-                newSha = prop.Value.ToObject<string>();
+                newSha = json;
             }
 
             // Just propagate back what we had initially, as an optimization for HEAD and cases 
@@ -75,7 +71,13 @@ commits[0] is JObject obj &&
                 newSha = originalSha;
 
             if (newSha != null)
+            {
+                // Make sure the X-Sha matches what we have now.
+                if (response.Headers.TryGetValues("X-Sha", out _))
+                    response.Headers.Remove("X-Sha");
+
                 response.Headers.TryAddWithoutValidation("X-Sha", newSha);
+            }
 
             return response;
         }