Check the whitebox code out in RAM when there is room, else on disk

The whitebox read of a mapped repository now asks for a RAM backed checkout in
shared memory, which is faster and never written to disk.  The fetch already
falls back to the temp directory when shared memory is missing or tight on
space, so a low memory host still works.  The checkout is shallow, read only,
and removed the moment the read finishes either way, and only the routes and
sinks it yields are kept, never the source.

Author: hackrange

app/core/external.py

@@ -1973,7 +1973,9 @@ def _whitebox(scan_id: int, asset_id: int, host: str) -> dict:
         entry = {"repository": name, "branch": branch, "routes": 0,
                  "host_in_code": [], "error": ""}
         try:
-            with recheck.branch_tip(connection, name, branch) as (root, why):
+            # Prefer a RAM backed checkout (/dev/shm): faster and never on disk.
+            # branch_tip falls back to the temp directory when RAM is tight.
+            with recheck.branch_tip(connection, name, branch, in_memory=True) as (root, why):
                 if root is None:
                     entry["error"] = why or "could not fetch it"
                 else: