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