blame: improve diagnosis for "--reverse NEW"

"git blame --reverse OLD..NEW -- PATH" tells us to start from the
contents in PATH at OLD and observe how each line is changed while
the history develops up to NEW, and report for each line the latest
commit up to which the line survives in the original form.

If you say "git blame --reverse NEW -- PATH" by mistake, we complain
about the missing OLD, but we phrased it as "No commit to dig down
to?"  In this case, however, we are digging up from OLD, so say so.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

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