Show commit message on the per-file review pages.

When reviewing changes the review may be interrupted for any number
of reasons. When continuing the review process the review might
have forgotten the content of the commit message.

This change shows the commit message at the per-file review page.
The area for the commit message is limited in order to preserve the
space for the diff view(s). If the commit message doesn't fit into
the commit message area a vertical/horizontal scrollbar will appear.

The commit message for the patch set to the right side (the "new
version") of the PatchScreen is cached to avoid one RPC every time
the patch screen switches to next/prev file.

Bug: issue 426
Bug: issue 680
Change-Id: I8db497e44cadab6be8895e1c477516d3b3b2d9c2
Signed-off-by: Sasa Zivkov <sasa.zivkov@sap.com>
Signed-off-by: Shawn O. Pearce <sop@google.com>
9 files changed