http-push: replace strcat with xsnprintf

http-push: replace strcat with xsnprintf

We account for these strcats in our initial allocation, but
the code is confusing to follow and verify. Let's remember
our original allocation length, and then xsnprintf can
verify that we don't exceed it.

Note that we can't just use xstrfmt here (which would be
even cleaner) because the code tries to grow the buffer only
when necessary.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>