Makefile: fix misdetection of relative pathnames

The installation rules wanted to differentiate between a template_dir that is given as an absolute path (e.g. /usr/share/git-core/templates) and a relative one (e.g. share/git-core/templates) but it was done by checking if $(abspath $(template_dir)) and $(template_dir) yield the same string. This was wrong in at least two ways. * The user can give template_dir with a trailing slash from the command line to invoke make or from the included config.mak. A directory path ought to mean the same thing with or without such a trailing slash but use of $(abspath) means an absolute path with a trailing slash fails the test. * Versions of GNU make older than 3.81 do not have $(abspath) to begin with. This changes the detection logic to see if the given path begins with a slash. Signed-off-by: Junio C Hamano <gitster@pobox.com>