`spr land` requires `git` at least at version 2.29.0

[Hardy Jones]

From #61

• Uses the --no-write-fetch-head option which was only introduced in 2.29.0.

And later in the issue from @sven-of-cord:

• we could query what version of git is installed, and not include the --no-write-fetch-head if it's not available. That means spr might overwrite your FETCH_HEAD file if you're on an older version of git, but that's probably better than not working at all.