builtin-fast-export: Only output a single parent per line

builtin-fast-export: Only output a single parent per line

According to the git-fast-import man-page, you can only put a single
committish per merge: line, like this:

  merge :10
  merge :11

However, git-fast-export puts all parents on a single line, like this:

  merge :10 :11

This changes fast-export to output a single parent per line.  Otherwise
neither git-fast-import nor bzr-fast-import can read its output.

[jc: fix-up to remove excess LF in the output that makes fast-import barf]

Signed-off-by: Pieter de Bie <pdebie@ai.rug.nl>
Signed-off-by: Junio C Hamano <gitster@pobox.com>