We can eliminate permission issues by not touching `~/.gitconfig` at
all.

Fixes #17067.

In this attempt we pretend that the "global" configs are in the
repository itself.

    $ XDG_CONFIG_HOME= HOME=$PWD git config --global section.key value
    $ XDG_CONFIG_HOME= HOME=$PWD git config --global section.key
    value
    $ git config --global section.key
    $ cat $PWD/.gitconfig
    [section]
      key = value

It is safe to override `HOME` for the entire script as only Git uses it.