x11-wm/picom: Improve "disable git" patch

Make it ready for upstream:
https://github.com/yshui/picom/pull/1520

While here remove unnecessary GH_PROJECT - it have default value.

PR: 295255
Approved by: José Maldonado <yukiteruamano@volfread.xyz> (maintainer, timeout 2 weeks)
Sponsored by: UNIS Labs
MFH: 2026Q2