Currently, it might happen that two different patches clash for the
same build number for the browsers. In this case, authors might
not even know that they need to rebaseline.
This patch starts adding a second line to `BUILD_NUMBER` files - the
signature and date of the `BUILD_NUMBER` change. These are guaranteed
to clash, so it should not be possible to land patches without
re-baselining them.
This changes `export.sh` to export diffs instead of patches.
`export.sh` now does not touch working directory, thus not
triggering clean builds and not intefering with an on-going
build.
