git-gui: Additional early feature development.

git-gui: Additional early feature development.

 * Run refresh before diff-index.
 * Load saved commit message during rescan.
 * Save current commit message (if any) during quit.
 * Add Signed-off-by line to commit buffer.
 * Batch update-index invocations through --stdin.
 * Better highlight which file is in the diff viewer.
 * Key bindings for signoff, check-in all and commit.
 * Improved formatting of status table within source.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>