git-gui: Reimplement and enhance auto-selection of diffs.

git-gui: Reimplement and enhance auto-selection of diffs.

Generalize the next_diff system, and implement auto-reselection
for merge tool resolution and reshow_diff. Also add auto-selection
of diffs after rescan, if no diff is already selected.

New auto-select rules:

- Rescan auto-selects the first conflicting file, or if none
  a modified tracked file, if nothing was selected previously.
- Resolving a conflict auto-selects the nearest conflicting
  file, or nothing if everything is resolved.
- Staging the last remaining hunk auto-selects the nearest
  modified staged file.
- Staging a file through its icon auto-selects the nearest file.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>