For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:tlusk:devtools/15.6/home:tlusk:devtools.repo
zypper refresh
zypper install git-delta
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:tlusk:devtools/15.5/home:tlusk:devtools.repo
zypper refresh
zypper install git-delta
For 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:tlusk:devtools/15.4/home:tlusk:devtools.repo
zypper refresh
zypper install git-delta