Set the probe branch upstream with push -u, and log why when it fails

Setting the upstream separately depended on the push having created
refs/remotes/origin/probe-main, which it did not on the CI runners. The
case then failed on its own positive control with nothing in the log
explaining it, so the fix was guesswork twice. push -u does both, and
the control now carries the push output when it fires.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: vedant-simulacrum

tests/install-matrix.sh

@@ -732,16 +732,18 @@ if want doctor-no-mutate; then
     # the destructive pairing, set locally so the case does not depend on the operator's config
     git -C "$T/work" config fetch.prune true
     git -C "$T/work" config fetch.pruneTags true
-    git -C "$T/work" push -q origin probe-main:refs/heads/probe-main 2>/dev/null
-    git -C "$T/work" branch -q --set-upstream-to=origin/probe-main 2>/dev/null
+    # push -u in one step. Setting the upstream separately depended on the push having created
+    # refs/remotes/origin/probe-main, which it did not do on the CI runners, and the case then
+    # failed on its own control with no way to see why from the log.
+    git -C "$T/work" push -u origin probe-main > "$T/push.log" 2>&1 || true
     git -C "$T/work" tag -a v9.9.9-local -m "never pushed" 2>/dev/null
     e=""
     # Two positive controls. Without them the case passes on any machine where the tag was never
     # created or where --drift declined to run, which is the shape of every fake green here.
     git -C "$T/work" rev-parse -q --verify refs/tags/v9.9.9-local >/dev/null 2>&1 \
       || e="$e; the probe tag was never created, so this case proves nothing"
     git -C "$T/work" rev-parse --symbolic-full-name '@{u}' >/dev/null 2>&1 \
-      || e="$e; no upstream, so the code path under test never runs"
+      || e="$e; no upstream, so the code path under test never runs [$(tr '\n' ' ' < "$T/push.log" 2>/dev/null | cut -c1-160)]"
     HOME="$T" VSTACK_DIR="$T/work" "$SRC/bin/doctor" --drift > "$T/out" 2>&1
     grep -q 'no vstack repo found' "$T/out" 2>/dev/null \
       && e="$e; --drift refused to run, so it never reached the fetch this case is about"