meson: fix git ls-files invocations during rebase

meson: fix git ls-files invocations during rebase

Normally ls-files prints the full path to files from the repo root. But when
$GIT_WORK_TREE is set, ls-files prints paths relative to the current
directory. When rebasing, $GIT_WORK_TREE is set in the commands executed from
'rebase -x'. This causes problems if meson config is touched and the meson
reconfigures itself. ($GIT_WORK_TREE shouldn't be relevant, since the paths that
ls-files reports don't depend on the work tree, but whatever.) Let's unset
GIT_WORK_TREE to avoid the issue.

$ (cd test; git --git-dir=$PWD/../.git ls-files ':/test/dmidecode-dumps/*.bin')
test/dmidecode-dumps/HP-Z600.bin
test/dmidecode-dumps/Lenovo-ThinkPad-X280.bin
test/dmidecode-dumps/Lenovo-Thinkcentre-m720s.bin

$ (cd test; GIT_WORK_TREE=$PWD/.. git --git-dir=$PWD/../.git ls-files ':/test/dmidecode-dumps/*.bin')
dmidecode-dumps/HP-Z600.bin
dmidecode-dumps/Lenovo-ThinkPad-X280.bin
dmidecode-dumps/Lenovo-Thinkcentre-m720s.bin

Fixes #18148.