http-push: fix off-by-path_len

http-push: fix off-by-path_len

When getting the result of remote_ls(), we were advancing the variable
"path" to the relative path inside the repository.

However, then we went on to malloc a bogus amount of memory: we were
subtracting the prefix length _again_, quite possibly getting something
negative, which xmalloc() interprets as really, really much.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>