Makefile: fix misdetection of relative pathnames

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>