luci.mk: rework logic to determine current branch
luci.mk: rework logic to determine current branch

In case `git branch --remote` yields no result, fall back to `git branch`
without `--remote` since it is possible that the local branch has not
been pushed yet.

Also simplify extraction of the branch name by not passing `--verbose`
and utilizing variable substitutions instead of sed expressions.

Signed-off-by: Jo-Philipp Wich <bd73d35759d75cc215150d1bbc94f1b1078bee01@mein.io>

hnyman/luci,openwrt/luci,hnyman/luci,openwrt/luci,openwrt/luci,hnyman/luci,hnyman/luci,openwrt/luci,openwrt/luci,openwrt/luci,hnyman/luci,openwrt/luci,hnyman/luci,hnyman/luci,hnyman/luci,openwrt/luci