builtin-reflog.c: don't install new reflog on write failure

builtin-reflog.c: don't install new reflog on write failure

When expiring reflog entries, a new temporary log is written which contains
only the entries to retain. After it is written, it is renamed to replace
the existing reflog. Currently, we check that writing of the new log is
successful and print a message on failure, but the original reflog is still
replaced with the new reflog even on failure. This patch causes the
original reflog to be retained if we fail when writing the new reflog.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>