git-gui: Add keyboard shortcuts for search and goto commands in blame view.

git-gui: Add keyboard shortcuts for search and goto commands in blame view.

Use forward-slash or Control-S to bring up the search dialog.
In the blame view, Enter or 'n' jump to the next selected region while
Shift-Enter or Shift-n will jump to the previous selected region.
Within the search control, hitting Enter will now jump to the next matching
region.

Signed-off-by: David Fries <David@Fries.net>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>