blame: optionally track line fingerprints during fill_blame_origin()

blame: optionally track line fingerprints during fill_blame_origin()

fill_blame_origin() is a convenient place to store data that we will use
throughout the lifetime of a blame_origin.  Some heuristics for
ignoring commits during a blame session can make use of this storage.
In particular, we will calculate a fingerprint for each line of a file
for blame_origins involved in an ignored commit.

In this commit, we only calculate the line_starts, reusing the existing
code from the scoreboard's line_starts.  In an upcoming commit, we will
actually compute the fingerprints.

This feature will be used when we attempt to pass blame entries to
parents when we "ignore" a commit.  Most uses of fill_blame_origin()
will not require this feature, hence the flag parameter.  Multiple calls
to fill_blame_origin() are idempotent, and any of them can request the
creation of the fingerprints structure.

Suggested-by: Michael Platings <michael@platin.gs>
Signed-off-by: Barret Rhoden <brho@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>