browse/snapshot_context: Ensure pull request branches can be browsed

While pull request branches are hidden by default in Web UI, they should
remain browsable.

That commit fixes the error that was previously raised when attempting
to browse such a branch.

swh/web/browse/snapshot_context.py

@@ -85,6 +85,9 @@ def _get_branch(branches, branch_name, snapshot_id):
             branches_from=branch_name,
             branches_count=1,
             target_types=["revision", "alias"],
+            # pull request branches must be browsable even if they are hidden
+            # by default in branches list
+            branch_name_exclude_prefix=None,
         )
         snp_branch, _, _ = process_snapshot_branches(snp)
         if snp_branch and snp_branch[0]["name"] == branch_name:

swh/web/tests/browse/views/test_origin.py

@@ -1297,3 +1297,22 @@ def test_pull_request_branches_filtering(client, origin):
         client, url, status_code=200, template_used="browse/branches.html"
     )
     assert_not_contains(resp, "refs/pull/")
+
+
+@given(origin_with_pull_request_branches())
+def test_browse_pull_request_branch(client, archive_data, origin):
+    snapshot = archive_data.snapshot_get_latest(origin.url)
+    pr_branch = random.choice(
+        [
+            branch
+            for branch in snapshot["branches"].keys()
+            if branch.startswith("refs/pull/")
+        ]
+    )
+    url = reverse(
+        "browse-origin-directory",
+        query_params={"origin_url": origin.url, "branch": pr_branch},
+    )
+    check_html_get_response(
+        client, url, status_code=200, template_used="browse/directory.html"
+    )