Fix external git diff-files being called repeatedly for the same file (Issue #530) - #599

… (issue gitx#530)

Answer the commit view from the diff it was last given while nothing it
was read from has changed, instead of starting another git for a file
that has not moved.

Staging one file re-renders the commit view three times:
- once as the selection moves on, and
- once for each of the two index refreshes that follow.
Each render asks for the diff of the file now on screen, so the same
`diff-files -U3 -- <path>` ran three times over an untouched file,
which is what issue gitx#530 reports seeing in the log.

The refresh output cannot tell whether a file's content changed,
only how it is staged, so the file and the index are stat'ed instead:
size and modification time to the nanosecond, alongside the path, the
tree being compared against and the requested context.

- Read a diff through a one entry cache keyed on all of that
- Fall back to reading it every time when the index cannot be located,
  since then there is nothing to notice a change against
- Cover both halves: a diff nothing could have changed is asked for
  once, and an edit, a rewritten index, a different context size or an
  amend are each asked for again