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.

A convenience function prefix_filename() can concatenate two paths
to form a path that points at somewhere outside the work tree.
Use it in these codepaths instead.

Signed-off-by: Junio C Hamano <gitster@pobox.com>