match: introduce explicitdir and traversedir match.dir is currently called in two different places: (1) noting when a directory specified explicitly is visited. (2) noting when a directory is visited during a recursive walk. purge cares about both, but commit only cares about the first. Upcoming patches will split the two cases into two different callbacks. Why bother? Consider a hypothetical extension that can provide more efficient walk results, via e.g. watching the filesystem. That extension will need to fall back to a full recursive walk if a callback is set for (2), but not if a callback is only set for (1).

'a' in d: True
d['a']: va
'b' in d: True
d['b']: vb
'c' in d: True
d['c']: vc
'd' in d: True
d['d']: vd
'a' in d: False
'b' in d: True
d['b']: vb
'c' in d: True
d['c']: vc
'd' in d: True
d['d']: vd
'e' in d: True
d['e']: ve
'b' in d: True
d['b']: vb2
'c' in d: True
d['c']: vc2
'd' in d: True
d['d']: vd
'e' in d: False
'f' in d: True
d['f']: vf