t0213: skip ancestry tests under user-mode emulation

The tests added in 3c8c638df6 (t0213: add trace2 cmd_ancestry tests,
2026-02-13) expect the cmd_ancestry event to name "test-tool" and
"git". On Linux those names come from the "comm" field of
/proc/<pid>/stat. Under user-mode emulation (e.g. qemu-user) /proc
reports the emulator ("qemu-riscv64") instead, so the event is still
emitted, the TRACE2_ANCESTRY probe enables the tests, and tests 2-5
fail even though they pass on native riscv64.

Require the probe to see "test-tool" in the ancestry of a test-tool
spawned from test-tool, so the tests skip when the names are unreliable.

Cc: Matthew John Cheetham <mjcheetham@outlook.com>
Signed-off-by: Jamie Magee <jamie.magee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0213-trace2-ancestry.sh

@@ -31,12 +31,15 @@ PATH="$TTDIR:$PATH" && export PATH
 # no cmd_ancestry event is emitted.  We detect this at runtime and
 # skip the format-specific tests accordingly.
 
-# Determine if cmd_ancestry is supported on this platform.
+# Enable these tests only when cmd_ancestry reports real process names.
+# The procinfo stub emits no event; under user-mode emulation (e.g.
+# qemu-user) /proc reports the emulator, not the guest. Spawn test-tool
+# from test-tool and require "test-tool" in the child's ancestry.
 test_expect_success 'detect cmd_ancestry support' '
 	test_when_finished "rm -f trace.detect" &&
 	GIT_TRACE2_BRIEF=1 GIT_TRACE2="$(pwd)/trace.detect" \
-		test-tool trace2 001return 0 &&
-	if grep -q "^cmd_ancestry" trace.detect
+		test-tool trace2 004child test-tool trace2 001return 0 &&
+	if grep -q "^cmd_ancestry.*test-tool" trace.detect
 	then
 		test_set_prereq TRACE2_ANCESTRY
 	fi