make_chroot: don't enable git-prompt via bashcomp

Newer git ebuilds have decided that the "git-prompt" script isn't really
bash completion so stopped installing it via that mechanism. Instead it
installed it started installing it in /usr/share/docs which gets
compressed by default and the path is based on ebuild version. The path
changed again in 1.9.3 to /usr/share/git and didn't compress it so that
makes it actually possibly usable but 1.9.3 or later isn't stable yet.

We can re-enable it the next time git gets updated but not worth fussing
over the current brokenness right now.