Dgit.pm: printcmd: Print in one go

Dgit.pm: printcmd: Print in one go

Sometimes this message is used to report failure of `git fetch' etc.
But when such operations fail, they may interleave output from the
remote git, which typically prints
  fatal: The remote end hung up unexpectedly
(referring to the local end as the `remote' end, from its point
of view), after the the local end printed its message and failed.

This sets up a race between Dgit.pm's printcmd and the message from
the remote.  If we're really unlucky, the remote's message interrupts
the output from printcmd.

We can avoid the within-line interleaving - at least, the interruption
of our message - by printing the whole message in one go, so do that.
(Strictly, the approach we use here only ensures that the message
appears in a single write(2) call if it fits in the stdio buffer.)

One of our test cases (tagupl) depends on this message not being split
and has a small chance of failing without this fix.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>