tags: support setting hgtags fnodes cache entries
An upcoming patch will teach bundle2 to transfer .hgtags fnodes
values. To support this, we need to support inserting values into
the cache.
Add functionality to do that.
'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
'b' in d: False
'c' in d: False
'd' in d: False
'e' in d: False
'f' in d: False