Prevent diff view nav links from overflowing the header

A long change subject together with a long file path made the header's
content wider than the header, pushing the Prev/Up/Next links outside
the visible area. `.headerLeft` is a flex item and could not shrink
below its content size, because flex items default to `min-width: auto`.

Set `min-width: 0` on `.headerLeft` so that it can shrink, and give
`.headerSubject` a high `flex-shrink` so that it absorbs the deficit
before the file path and the nav links give up any space. Make the
subject a direct flex item so that `text-overflow: ellipsis` applies to
it.

This regressed in 1f7cc7c951, which turned `.headerLeft` into a flex
row. Before that the left group was an inline formatting context, so
long content wrapped onto its own line instead of overflowing.

Release-Notes: Fix Prev/Up/Next links overflowing the diff view header when the change subject and file path are long.
Bug: Issue 542859659
Screenshot: https://postimage.me/image/KJVgP (before)
Screenshot: https://postimage.me/image/KJVE9 (after)
Change-Id: I87a3928112f22f68361fcd2d438dc240742e088a