gitweb: Parse two-line from-file/to-file diff header in git_patchset_body

gitweb: Parse two-line from-file/to-file diff header in git_patchset_body

Parse two-line from-file/to-file unified diff header in
git_patchset_body directly, instead of leaving pretty-printing to
format_diff_line function.  Hashes as from-file/to-file are replaced
by proper from-file and to-file names (from $diffinfo); in the future
we can put hyperlinks there.  This makes possible to do blobdiff with
only blobs hashes.

The lines in two-line unified diff header have now class "from_file"
and "to_file"; the style is chosen to match previous output (classes
"rem" and "add" because of '-' and '+' as first character of patch
line).

Signed-off-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>