git-gui: Improve keyboard traversal in dialogs.

git-gui: Improve keyboard traversal in dialogs.

When we are in a dialog such as the new branch dialog or our options
dialog we should permit the user to traverse around through the available
widgets with their Tab/Shift-Tab key combinations.  So in any single
line text field where we don't want tab characters to actually be
inserted into the value rebind Tab and Shift-Tab to honor what the
tk_focusPrev and tk_focusNext scripts recommend.

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