Makefile: Do not install a copy of 'git' in $(gitexecdir)

Makefile: Do not install a copy of 'git' in $(gitexecdir)

There is already a copy in $(bindir). A subsequent patch will enable git
to derive the exec-path from its invocation path. If git is invoked
recursively, the first invocation puts the exec-path into PATH, so that
the recursive invocation would find the instance in the exec-path. This
second instance would again try to derive an exec-path from its invocation
path, but would base its result on the wrong "bindir".

We do install the copy of git first, but remove it later, so that we can
use it as the source of the hardlinks for the builtins.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>