Merge branch 'vd/scalar-to-main'

Merge branch 'vd/scalar-to-main'

Hoist the remainder of "scalar" out of contrib/ to the main part of
the codebase.

* vd/scalar-to-main:
  Documentation/technical: include Scalar technical doc
  t/perf: add 'GIT_PERF_USE_SCALAR' run option
  t/perf: add Scalar performance tests
  scalar-clone: add test coverage
  scalar: add to 'git help -a' command list
  scalar: implement the `help` subcommand
  git help: special-case `scalar`
  scalar: include in standard Git build & installation
  scalar: fix command documentation section header