gitweb: Fix and simplify "split patch" detection

gitweb: Fix and simplify "split patch" detection

There are some cases when one line from "raw" git-diff output (raw
format) corresponds to more than one patch in the patchset git-diff
output; we call this situation "split patch". Old code misdetected
subsequent patches (for different files) with the same pre-image and
post-image as fragments of "split patch", leading to mislabeled
from-file/to-file diff header etc.

Old code used pre-image and post-image SHA-1 identifier ('from_id' and
'to_id') to check if current patch corresponds to old raw diff format
line, to find if one difftree raw line coresponds to more than one
patch in the patch format.  Now we use post-image filename for that.
This assumes that post-image filename alone can be used to identify
difftree raw line.  In the case this changes (which is unlikely
considering current diff engine) we can add 'from_id' and 'to_id'
to detect "patch splitting" together with 'to_file'.

Because old code got pre-image and post-image SHA-1 identifier for the
patch from the "index" line in extended diff header, diff header had
to be buffered.  New code takes post-image filename from "git diff"
header, which is first line of a patch; this allows to simplify
git_patchset_body code.  A side effect of resigning diff header
buffering is that there is always "diff extended_header" div, even
if extended diff header is empty.

Alternate solution would be to check when git splits patches, and do
not check if parsed info from current patch corresponds to current or
next raw diff format output line.  Git splits patches only for 'T'
(typechange) status filepair, and there always two patches
corresponding to one raw diff line.  It was not used because it would
tie gitweb code to minute details of git diff output.

While at it, use newly introduced parsed_difftree_line wrapper
subroutine in git_difftree_body.

Noticed-by: Yann Dirson <ydirson@altern.org>
Diagnosed-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>