* fix(scripts): find Git Bash under a PortableGit mingw64 layout
`scripts/run-with-git-bash.cmd` derives `bash.exe` from `git.exe`'s parent,
which only holds for the Git for Windows `<root>/cmd` and `<root>/bin`
layouts. PortableGit and GitHub Desktop ship `git.exe` in
`<root>/mingw64/bin`, so the wrapper looked for `<root>/mingw64/bin/bash.exe`
and reported "Could not locate Git for Windows Bash" even though
`<root>/bin/bash.exe` exists. Every Makefile recipe that runs a repository
shell script failed on those hosts.
Resolve the shell under the Git installation root instead, and mirror the rule
in `backend/tests/support/shell.py` (documented as mirroring the wrapper) so
the shell-script suites stop skipping on the same hosts.
* test(backend): pin both Git layouts in one shell-discovery suite
Review follow-up for the PortableGit probe:
- `_git_roots` now probes `git.exe`'s parent and grandparent unconditionally —
exactly the two directories the Makefile wrapper checks — so the batch probe
and the Python mirror cannot drift; the docstring records that the first
`mingw64` candidate normally misses and is kept only to mirror it.
- fold the candidate-layout cases and the Windows-only wrapper case into
`tests/test_support_shell.py`, which already owns this module's unit tests,
instead of a second file with a duplicate `_touch` helper.
- report the wrapper's stdout in the failure message: its diagnostic goes to
stdout, so `stderr` was empty on the discovery-failure path.
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