Make reflog query '@{1219188291}' act as '@{2008.8.19.16:24:51.-0700}'

Make reflog query '@{1219188291}' act as '@{2008.8.19.16:24:51.-0700}'

As we support seconds-since-epoch in $GIT_COMMITTER_TIME we should
also support it in a reflog @{...} style notation.  We can easily
tell this part from @{nth} style notation by looking to see if the
value is unreasonably large for an @{nth} style notation.

The value 100000000 was chosen as it is already used by date.c to
disambiguate yyyymmdd format from a seconds-since-epoch time value.

A reflog with 100,000,000 record entries is also simply not valid.
Such a reflog would require at least 7.7 GB to store just the old
and new SHA-1 values.  So our randomly chosen upper limit for @{nth}
notation is "big enough".

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>