Revert "git: fix build on FreeBSD"

This reverts commit ab98d72fad2c657836d36fe0ac2f7dac54f1b778, which
caused the size of the Git package in the store to increase from 18
MiB to 190 MiB.
git: Properly use symlinks instead of hard links

Git's Makefile has a NO_INSTALL_HARDLINKS flag to produce symlinks
instead of hard links.  However, it still produces hard links between
$out/bin and $out/libexec, hence the patch.

Also, update Git to 1.8.2.1.