gitweb: Add invisible hyperlink to from-file/to-file diff header

gitweb: Add invisible hyperlink to from-file/to-file diff header

Change replacing hashes as from-file/to-file with filenames from
difftree to adding invisible (except underlining on hover/mouseover)
hyperlink to from-file/to-file blob.  /dev/null as from-file or
to-file is not changed (is not hyperlinked).

This makes two-file from-file/to-file unified diff header parsing in
git_patchset_body more generic, and not only for legacy blobdiffs.

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