git: Wire up Git v2.40 execution environment
Wire up the Git v2.40 execution environment so that we can toggle between the v2.39 and v2.40 release tracks. Major new features that we're interested in include:
- 'cat-file' gains mailmap support for its '--batch-check' and '-s'
options.
- "git check-attr" learned to take an optional tree-ish to read the
.gitattributes file from.
There are naturally various other new features, bugfixes and performance improvements.
The new Git v2.40 binary is protected by a feature flag.
Closes #4982 (closed)