Fix prematurely re-drawing graph for matrix build.

I still don't fully understand why the right border gets cropped, but
playing with DOM manipulation window, it seems like the cell-level
"border-right: none" is winning over the table level "border: 1px solid
talks about the border conflict resolution specified in
http://www.w3.org/TR/CSS2/tables.html#border-conflict-resolution

What this fix does is to set the cell-level right border for pane
tables. I realize that this might break table nesting inside another
table.

CLOUDBEES-2124.

configurations in the matrix.

instead of the default HTML table look. Also moving the
"Configuration Matrix" title as a part of the table, to gain some space.

- doc update

Don't call SCM#calcPollingingBaseline if SCM#checkout fails

In the JavaDocs for SCM#calcRevisionsFromBuild, if says:
This method is called after source code is checked out for the given build (that is, after SCM#checkout has finished successfully.)

This appears to make sense; after you have a known SCM state, you calculate the revisions.  It doesn't explicitly state it, but it
implies that it won't be called if checkout failed.  In those cases, the state may well not be known, and it doesn't make sense to
attempt to calculate the revisions.

The behavior in AbstractProject#checkout has been to always call calcPollingBaseline, regardless of whether checkout failed or
succeeded.  This is causing misleading error output for the mercurial plugin.  With this change, AbstractProject will only do the
calculations when checkout succeeds.