Makefile: Normalize $(bindir) and $(gitexecdir) before comparing

Makefile: Normalize $(bindir) and $(gitexecdir) before comparing

The install target needs to check whether the user has opted to make
$(gitexecdir) equal to $(bindir). It did so by a straight string
comparison. Since we are going to allow a relative $(gitexecdir), we have
to normalize paths before comparison, which we do with $(cd there && pwd).

The normalized paths are stored in shell variables. These we can now
reuse in the subsequent install statements, which conveniently shortens
the lines a bit.

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