gitweb: git_blobdiff_plain is git_blobdiff('plain')

gitweb: git_blobdiff_plain is git_blobdiff('plain')

git_blobdiff and git_blobdiff_plain are now collapsed into one
subroutine git_blobdiff, with format (currently 'html' which is
default format corresponding to git_blobdiff, and 'plain'
corresponding to git_blobdiff_plain) specified in argument.

blobdiff_plain format is now generated either by git-diff-tree
or by git-diff.  Added X-Git-Url: header.  From-file and to-file name
in header are corrected.

Note that for now commitdiff_plain does not detect renames
and copying, while blobdiff_plain does.

While at it, set expires to "+1d" for non-textual hash ids.

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