OpenWRT + https + git
A lot of the times since routers are very limited in storage space, many of the applications in the repos are just the bare bones of that application with many of the features turned off to minimize space consumption.
Git is no different. The git binary in the opkg repo was compiled without https support.
To fix this, you are probably going to need to cross-compile your own version of git with both curl and expat enabled... which is fun and exciting. (if interested check out http://buildroot.uclibc.org/ I use mips-buildroot-linux-uclibc as my cross compiler for my TP-Link WDR 3500)
An alternative would be to try to use a different protocol (either ssh or git) if practical. Those protocols are better supported by git anyhow.