wireprotoserver: access headers through parsed request
Now that we can access headers via the parsed request object, let's
do that.
Since the new object uses bytes, hyphens, and is case-insensitive, a
bit of code around normalizing values has been removed. I think
the new code is much more intuitive because it more closely matches
what is going out over the wire.
Differential Revision: https://phab.mercurial-scm.org/D2743
$ hg init
$ echo 123 > a
$ hg add a
$ hg commit -m "first" a
$ mkdir sub
$ echo 321 > sub/b
$ hg add sub/b
$ hg commit -m "second" sub/b
$ cat sub/b
321
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ cat sub/b 2>/dev/null || echo "sub/b not present"
sub/b not present
$ test -d sub || echo "sub not present"
sub not present