Support meson dist when getting project versions from VCS

[joukewitteveen]

Together with #12782, this allows versioning via git (#688) as follows.

meson.build:

project('tig', 'c',
  version : run_command('version.sh', 'get_vcs').stdout.strip())
meson.add_dist_script('version.sh', 'set_dist', meson.project_version())

version.sh:

#!/bin/sh
if [ "$1" = "get_vcs" ]; then
  git -C "$MESON_SOURCE_ROOT" describe --always --dirty
elif [ "$1" = "set_dist" ]; then
  $MESONREWRITE --sourcedir="$MESON_PROJECT_DIST_ROOT" kwargs set project / version "$2"
else
  exit 1
fi

Without #12782, the rewriting will not work, so that one needs to be merged before the current pull request.