Handle blaming beyond the creation of file more gracefully

Handle blaming beyond the creation of file more gracefully

Currently when you ask to re-blame from the parent of a
commit that created the file, blame_read_file will complain
that it cannot get the file contents ("No blame exist").

At the time we try to read the file, it is too late to abort
the operation, as we have already changed to the new blame
view. However, we can detect this situation early by
limiting the selection of the parent revision to the
particular path of interest: if it returns a parent even
with path-limiting, then we know the path exists; if not,
then we know it doesn't.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Jonas Fonseca <fonseca@diku.dk>