the family root's bootstrap.py: git's update killed by a signal (exit 129 and above) gives the credential advice and family bootstrap ok #998

What happens

Lane 5's probe P3 on #954, git 2.43.0:

• The state: members/IRRS's own .gitmodules gives its code leg a bare branch, a key with no value.
• Git: submodule--helper died of signal 11. Its update exits 139 and goes on to IRSS.
• Bootstrap (--skip-members): FETCH INCOMPLETE (git exit 139), the credential advice "If the members are PRIVATE, set SHAPE_LEGS_TOKEN ...", and family bootstrap ok, exit 0.

A signal is never a missing credential's. The run should not end ok or give the advice.

The behaviour is already there at #889's 29083a5f, and #954 (#892) does not change it. #954 asks where git stopped only on exit 128 (GIT_DIED), and the issue's lines exit exactly 128. Lane 5 listed this as checklist candidate 1 on #954.

Direction

• Where git's update ends by a signal, end the run FAILED, naming the signal and the exit code, with no credential advice. For example: UPDATE KILLED, and not for want of a credential: git's update ended on signal 11 (exit 139).
• Two shapes to read:
  • an exit code above 128, which git gives where a child it ran died of signal N (128 + N);
  • a negative returncode, where git itself was killed (Python's -N).
• Read only the exit code, never git's words, as bootstrap.py (family root): after #879, two states still end family bootstrap ok beside git's exit 128: a stray gitlink inside a member's own index, and a member's gitlink at a commit no store holds #892 does.
• the family root's bootstrap.py: an (ignored) line, for any code or signal, resets the depth fully, so a member's own exit 2 after a - one or two makes below, or after || true and a -, reads exited 2, and the limits name the -k let-fail shape (#890) #891 read make's (ignored) lines for a code or a signal in step (b). This is the same question for step (a).

Verify

• P3's state on git 2.43: exit non-zero, the signal named, no advice and no ok.
• A portable row, since a newer git may not crash on a bare branch (tests/test_doctor_legs_members_next.py already skips 6 rows where 2.43 crashes on one): a wrapper git first on PATH that runs the real git for everything but submodule update, and for that one exits 139 or kills itself with SIGSEGV. Both shapes are then named, and neither gives the advice.
• The credential control (no credential, pin moved on, exit 128) keeps the advice and ok.

Found by lane 5's review of #954 (checklist candidate 1, issuecomment-6253384652). Follows #892 (#954).

Parent: #459.

Milestone: v1.2.0.

Not claimed.

Lane: openreposhape-4 (openRepoShape-4)