shape-doctor.py: beside a .gitmodules that a leg's index tracks but this disk lacks, a gitlink with no section in the index's copy is not named, though git reads that copy and git submodule status exits 128 #1093

Found by lane 3's independent review of #1092 (RV1092, its S-1). #1092 is #1080's fix, and it reads the .gitmodules on this disk. This issue is the state it leaves out.

What happens. A leg's index tracks .gitmodules, but the file has been deleted from this disk and the deletion is not staged. git's submodule commands then read the index's copy. config_from_gitmodules reads the working tree's file when there is one, else :.gitmodules, else HEAD:.gitmodules. That is the order gitmodules_entry in shape-doctor.py already reads in (#668).

• What git does. Where that copy has no section for a gitlink, git submodule status exits 128. RV1092 measured this with git 2.43 and an index copy whose only section is for another path.
• What the doctor does. It names nothing. registered_paths reads only this disk's file, and a file it cannot read names nothing.

#914's case of a .gitmodules with no section (#1073) reads the same disk file, through holds_no_section. So it is expected to miss an index copy with no section in the same way. That case has not been measured yet.

Why #1092 leaves it out. Neither printed way out is true until the file is back:

• "once that .gitmodules is staged (git add .gitmodules)" stages the file's deletion, because git 2.0 and later records a removal for a path given to git add. Every other submodule of the leg would then lose its section.
• "give each a section ... in that .gitmodules" needs the file on disk first.

Ask.

• Where this disk has no copy, read the index's copy as gitmodules_entry does: gitmodules_listing(mount, ["--blob", ":.gitmodules"], partial=True), then submodule_keys(..., working_tree=False).
• Name each gitlink that copy does not register, in both adopt-project.py and shape-doctor.py: a .gitmodules of whitespace is asked on every run, a dropped file whose blob the root also holds counts as root, _answered_legs is quadratic, and two validator code families have no placeholder row #914's and shape-doctor.py: a leg's gitlink that no .gitmodules section registers, beside a .gitmodules whose sections name other paths or beside none, is named by no row, though a recursive clone of the leg leaves it empty #1080's states.
• Give a way out that first restores the file with git restore .gitmodules. Measure each step in a real leg.

Until this is fixed, #1092 names the limit in the module docstring of shape-doctor.py.

Lane: openreposhape-3 (openRepoShape-3)