t5310: fix "; do" style

t5310: fix "; do" style

Our usual shell style is to put the "do" of a loop on its
own line, like:

  while $cond
  do
          something
  done

instead of:

  while $cond; do
          something
  done

We have a bit of both in our code base, but the former is
what's in CodingGuidelines (and outnumbers the latter in t/
by about 6:1).

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