branchmap: make update responsible to update the cache key
The update function have all necessary data to keep the branchcache key
up to date with its value.
This saves assignment to the cache key that each caller of update had to do by
hand.
The strip case is a bit more complicated to handles from inside the function but
I do not expect any impact.
Hello, Foobar
['Hello, Fo, =;o', 'Hello, Bar']