Allow a relative builtin template directory.

Allow a relative builtin template directory.

In order to make git relocatable (i.e. not have the prefix compiled-in)
the template directory must depend on the location where this git instance
is found, which is GIT_EXEC_DIR.

The exec path is prepended only if the compiled-in default template
directory is to be used and that is relative. Any relative directories
that are specified via environment variable or the --exec-dir switch are
taken as is.

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