t7900: do not let `$HOME/.gitconfig` interfere with XDG tests

t7900: do not let `$HOME/.gitconfig` interfere with XDG tests

The XDG config tests for `git maintenance register/unregister`
create a fresh `$XDG_CONFIG_HOME/git/config` and expect git to use
that location. However, if `$HOME/.gitconfig` exists (which may
happen when test-lib.sh writes global config, e.g. to set
`safe.bareRepository`), git prefers `$HOME/.gitconfig` over the XDG
location, and the `maintenance.repo` entry ends up in the wrong
file.

This is an inherent consequence of setting global config in
test-lib.sh rather than adjusting individual tests: writing any
entry to `$HOME/.gitconfig` has side effects beyond the intended
setting, because the mere existence of that file changes which
global config location git prefers for all subsequent writes.
Individual per-test adjustments would not have this interaction.

Fix this by overriding `HOME` to a non-existent directory inside the
subshells that test XDG behavior. Since these subshells already
override `XDG_CONFIG_HOME`, they do not need `$HOME/.gitconfig` at
all, and the subshell scoping ensures the original `HOME` is
restored automatically.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>