wt-status: Split header generation into three functions
wt-status: Split header generation into three functions

Reorganize header generation so that all header text related to each
block is in one place.

This adds a function, but makes it easier to see what is generated in
each case. It also allows for easy tweaking of individual headers.

Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git