WARNING: Spoilers spoken here.
This time it was almost unanimous. It’s Kate’s braid.
Marty hit the nail head on in her comment. Kate seldom does anything for only one reason. That braid was just too convenient a handle by which to be dragged around, and Kate gives no edge to the bad guys. Even more important in that moment, and as many of you pointed out, a woman cutting her hair is a sign of mourning in many cultures.
Besides, shorter hair is easier to care for, and Kate practices pragmatism like she took a vow. Easy, expedient, practical, these are words by which she lives.
And let’s face it, she’s not the vainest person you ever met. Her self image is not bound up in her hair.
I like Laura’s comment about hunting, too. I visited the Purdey store in London back in the day. My ambition was to buy a pair of Purdey shotguns for my father, but he died before I could save up enough money. They were very nice to me in spite of my jeans and tennis shoes, and it was lovely to at least dream big there for a while.