Clear the linter, and fix the three things it found - #4

CI has been red on lint since the rewrite landed. Nineteen issues, and
three of them were real.

Page said it escaped every path segment and copied them across untouched.
That works until the first branch with a space in its name. It escapes
now.

GetJSON handed a 406 or a 410 to the JSON decoder and reported whatever
the decoder made of the HTML it got. Both statuses are routing answers,
so it says which road instead: noJSONHere, which had been written for
exactly this and never wired up.

github archive -o wrote to a file with a deferred close whose error went
nowhere, and a close is where the last of a write gets flushed. It closes
by hand and returns the error.

The rest is dead code and style. Two thirds of merge.go was a reflective
field-level merge that nothing calls: records are assembled by decoding
several blocks of one page into one struct, so there is nothing to merge.
What is left is the three provenance helpers that are used, and the file
is called provenance.go now because that is what it does. timeoutFor went
with it, since the byte-plane commands it named do get a context. Then a
De Morgan, two append loops, a tagged switch, an ineffectual assignment,
and a comment that still called railsNext by its old name.