blame: output porcelain "previous" header for each file

It's possible for content currently found in one file to
have originated in two separate files, each of which may
have been modified in some single older commit.  The
--porcelain output generates an incorrect "previous" header
in this case, whereas --line-porcelain gets it right.  The
problem is that the porcelain output tries to omit repeated
details of commits, and treats "previous" as a property of
the commit, when it is really a property of the blamed block
of lines.

Let's look at an example. In a case like this, you might see
this output from --line-porcelain:

  SOME_SHA1 1 1 1
  author ...
  committer ...
  previous SOME_SHA1^ file_one
  filename file_one
          ...some line content...
  SOME_SHA1 2 1 1
  author ...
  committer ...
  previous SOME_SHA1^ file_two
  filename file_two
          ...some different content....

The "filename" fields tell us that the two lines are from
two different files. But notice that the filename also
appears in the "previous" field, which tells us where to
start a re-blame. The second content line never appeared in
file_one at all, so we would obviously need to re-blame from
file_two (or possibly even some other file, if had just been
renamed to file_two in SOME_SHA1).

So far so good. Now here's what --porcelain looks like:

  SOME_SHA1 1 1 1
  author ...
  committer ...
  previous SOME_SHA1^ file_one
  filename file_one
          ...some line content...
  SOME_SHA1 2 1 1
  filename file_two
          ...some different content....

We've dropped the author and committer fields from the
second line, as they would just be repeats.  But we can't
omit "filename", because it depends on the actual block of
blamed lines, not just the commit. This is handled by
emit_porcelain_details(), which will show the filename
either if it is the first mention of the commit _or_ if the
commit has multiple paths in it.

But we don't give "previous" the same handling. It's written
inside emit_one_suspect_detail(), which bails early if we've
already seen that commit. And so the output above is wrong;
a reader would assume that the correct place to re-blame
line two is from file_one, but that's obviously nonsense.

Let's treat "previous" the same as "filename", and show it
fresh whenever we know we are in a confusing case like this.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

0 files changed, 0 insertions, 0 deletions