git: fix install of docs across file system boundaries

Since I build almost everything with HOMEBREW_BUILD_FROM_SOURCE=1 and with HOMEBREW_TEMP=/non-HOMEBREW_PREFIX/volume, upgrading `git` is a pain, since it fails to install docs across file system boundaries due to a single symlink.

There are no doubt other ways of doing this, [like untarring docs into `prefix/share`](https://github.com/Homebrew/homebrew/pull/14710#issuecomment-8341283), etc., but I thought this simple edit was descriptive of the issue.