Reimplements Git repository mirroring because Git is a special snowflake.
    
    etc/{README.md,pkgtool.usage}: updated.
    groups/221.native_packages_dev.group:cparser:${PKG_DISABLED}: disabled due to repository unavailability.
    groups/251.native_packages_lib.group:libfirm:${PKG_DISABLED}: disabled due to repository unavailability.
    midipix.env:${DEFAULT_BUILD_VARS}: adds MIRRORS_GIT.
    midipix.env:${DEFAULT_MIRRORS{,_GIT}}: split; mirror Git repositories via https://midipix.lucioillanes.de/repos_git/.
    pkgtool.sh:pkgtoolp_mirror_fetch(): call rtl_fetch_mirror_urls_git() vs. rtl_fetch_urls_git().
    pkgtool.sh:pkgtoolp_mirror_fetch(): symlink instead of downloading given package w/ parent package.
    pkgtool.sh:pkgtoolp_mirror{,_fetch}(): split archives vs. Git repositories directory name.
    subr.rtl/rtl_fetch.subr: factor out ${DEFAULT_GIT_ARGS}.
    subr.rtl/rtl_fetch.subr:rtl_fetch_mirror_urls_git(): initial implementation.
    subr.rtl/rtl_fetch.subr:rtlp_fetch_url_git(): always return on failure.
    subr.rtl/rtl_fetch.subr:rtlp_fetch_url_git(): correctly attempt to git-clone(1) from ${PKG_URL} and then via ${PKG_MIRRORS}.
    subr.rtl/rtl_fetch.subr:{rtlp_fetch_url_git,rtl_fetch_urls_git}(): cleanup.
    {subr/pkg_fetch_download.subr,vars/{gcc,python[23]{,_host}}.vars}: call rtl_fetch_urls_git() w/ ${PKG_NAME} and ${PKG_MIRRORS_GIT:-}.