subtree: drop support for git < 1.7

Suport for Git versions older than 1.7.0 (older than February 2010) was nice to have when git-subtree lived out-of-tree. But now that it lives in git.git, it's not necessary to keep around. While it's technically in contrib, with the standard 'git' packages for common systems (including Arch Linux and macOS) including git-subtree, it seems vanishingly likely to me that people are separately installing git-subtree from git.git alongside an older 'git' install (although it also seems vanishingly likely that people are still using >11 year old git installs). Not that there's much reason to remove it either, it's not much code, and none of my changes depend on a newer git (to my knowledge, anyway; I'm not actually testing against older git). I just figure it's an easy piece of fat to trim, in the journey to making the whole thing easier to hack on. "Ignore space change" is probably helpful when viewing this diff. Signed-off-by: Luke Shumaker <lukeshu@datawire.io> Signed-off-by: Junio C Hamano <gitster@pobox.com>