Rearrange startup to make gitk reloadable for development and testing.

Rearrange startup to make gitk reloadable for development and testing.

This patch moves the startup code into procedures so that the script
can be re-sourced during interactive development. This also permits the
script to be loaded as a library to permit unit-testing of gitk functions.

In the global namespace where gitk lives, variable and global are equivalent
commands except that variable can assign a value when initializing the
global variable while global just declares a name in the global table.

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