Fix misuse of prefix_path()

Fix misuse of prefix_path()

When DEFAULT_GIT_TEMPLATE_DIR is specified as a relative path,
init-db made it relative to exec_path using prefix_path(), which
is wrong.  prefix_path() is about a file inside the work tree.
There was a similar misuse in config.c that takes relative
ETC_GITCONFIG path. Noticed by Junio C Hamano.

We concatenate the paths manually. (prefix_filename() won't do
because it expects a prefix with a trailing '/'.)

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