blame: fix -L bounds checking bug

blame: fix -L bounds checking bug

Since inception, -LX,Y has correctly reported an out-of-range error when
Y is beyond end of file, however, X was not checked, and an out-of-range
X would cause a crash.  92f9e273 (blame: prevent a segv when -L given
start > EOF; 2010-02-08) attempted to rectify this shortcoming but has
its own off-by-one error which allows X to extend one line past end of
file.  For example, given a file with 5 lines:

  git blame -L5 foo  # OK, blames line 5
  git blame -L6 foo  # accepted, no error, no output, huh?
  git blame -L7 foo  # error "fatal: file foo has only 5 lines"

Fix this bug.

In order to avoid regressing "blame foo" when foo is an empty file, the
fix is slightly more complicated than changing '<' to '<='.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>