hggit: raise AmbiguousPrefixLookupError like mercurial
Mercurial passed from raising LookupError to a more specific AmbiguousPrefixLookupError in df0873ab5c14 (2018-04-13), which was released in v4.8 (2018-10-28).
This will, in time, allow to simplify a special handling code in TortoiseHg (see ba2dc2a07224 there).
Same as !69 (merged), by @muxator which was marked as “merged” by mistake.