[HUD][Test Insights] Better PR view for test counts (#7040)

* Change UI to get rid of nesting structure in favor of one big table.
This might make it harder to determine which file caused the overall
change in test times?
* Automatically select a sha to do base comparison with
  * Move merge base query that was originally in Dr. CI to named query

Some of the code in this PR isn't directly used here but will be used in
a different PR



https://torchci-git-csl-tibettertestcountsprview-fbopensource.vercel.app/pytorch/pytorch/commit/72e4786d1635681b8d053d0168c7d16b980e5124

Old
<img width="1679" height="520" alt="image"
src="https://github.com/user-attachments/assets/477b3023-abac-4c3d-a57b-500587e2d37c"
/>

New
<img width="1675" height="571" alt="image"
src="https://github.com/user-attachments/assets/2c3a3e55-f03d-4bec-a520-fa113d1e5a2c"
/>

Author: clee2000

torchci/clickhouse_queries/merge_bases/params.json

@@ -0,0 +1,7 @@
+{
+  "params": {
+    "shas": "Array(String)",
+    "repo": "String"
+  },
+  "tests": []
+}

torchci/clickhouse_queries/merge_bases/query.sql

@@ -0,0 +1,10 @@
+select
+    sha as head_sha,
+    merge_base,
+    merge_base_commit_date,
+from
+    merge_bases
+where
+    sha in {shas: Array(String)}
+    and merge_base_commit_date != 0
+    and repo = {repo: String}