Makefile: Fix invalid usage of GIT_INSTALL_DIR
When installing Git via make git, then we have two variables which
guide where things will be installed to: GIT_PREFIX is user-controlled
and determines where Git should be installed to, and GIT_INSTALL_DIR is
the default value in case GIT_PREFIX is not set. So ultimately, Git will
always be installed into GIT_PREFIX.
The rules we have for installing Git use GIT_INSTALL_DIR as target
though, so when one installs Git with a manually-chosen GIT_PREFIX, then
the target mismatches the location where Git actually ends up. This used
to work alright given that we never did anything with the target except
for executing make install in the Git project, but with the recent to
introduce a bundled Git version we started to touch(1) the target. This
thus now fails.
Fix this by using GIT_PREFIX instead of GIT_INSTALL_DIR.
Changelog: fixed