feat: add raw diff/patch endpoint for repository comparisons (#37632)

## Summary

Adds `GET
/repos/{owner}/{repo}/compare/{basehead}.{diffType:diff|patch}`,
mirroring the existing `/git/commits/{sha}.{diffType}` endpoint but for
comparisons between two arbitrary refs.

The new endpoint streams a raw unified diff or `git format-patch` output
between any two refs:

GET /repos/{owner}/{repo}/compare/main...feature.diff
GET /repos/{owner}/{repo}/compare/v1.0..v1.1.patch
GET /repos/{owner}/{repo}/compare/abc1234...def5678.diff

Resolves #5561, #13416 and #17165.

AI was used while creating this PR. Automated tests were added as per
the contribution policy.

---------

Co-authored-by: bircni <bircni@icloud.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

templates/swagger/v1_json.tmpl

@@ -8108,8 +8108,10 @@
     },
     "/repos/{owner}/{repo}/compare/{basehead}": {
       "get": {
+        "description": "By default returns JSON commit comparison information. The raw diff or patch can be\nrequested with the `output` query parameter set to `diff` or `patch` respectively.\n",
         "produces": [
-          "application/json"
+          "application/json",
+          "text/plain"
         ],
         "tags": [
           "repository"
@@ -8133,10 +8135,20 @@
           },
           {
             "type": "string",
-            "description": "compare two branches or commits",
+            "description": "compare two refs as `base...head` (or `base..head`); refs may be branches, tags, full or short SHAs, including branch names that contain slashes.",
             "name": "basehead",
             "in": "path",
             "required": true
+          },
+          {
+            "enum": [
+              "diff",
+              "patch"
+            ],
+            "type": "string",
+            "description": "return the raw comparison as `diff` or `patch` instead of JSON",
+            "name": "output",
+            "in": "query"
           }
         ],
         "responses": {

templates/swagger/v1_openapi3_json.tmpl

@@ -19468,6 +19468,7 @@
     },
     "/repos/{owner}/{repo}/compare/{basehead}": {
       "get": {
+        "description": "By default returns JSON commit comparison information. The raw diff or patch can be\nrequested with the `output` query parameter set to `diff` or `patch` respectively.\n",
         "operationId": "repoCompareDiff",
         "parameters": [
           {
@@ -19489,13 +19490,25 @@
             }
           },
           {
-            "description": "compare two branches or commits",
+            "description": "compare two refs as `base...head` (or `base..head`); refs may be branches, tags, full or short SHAs, including branch names that contain slashes.",
             "in": "path",
             "name": "basehead",
             "required": true,
             "schema": {
               "type": "string"
             }
+          },
+          {
+            "description": "return the raw comparison as `diff` or `patch` instead of JSON",
+            "in": "query",
+            "name": "output",
+            "schema": {
+              "enum": [
+                "diff",
+                "patch"
+              ],
+              "type": "string"
+            }
           }
         ],
         "responses": {