What happens if we have a hash collision

[Jesus Cea]

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With regular commands (like -r parameter, or "hg log", etc), Mercurial
only shows 48 bits of the hash.

According to birthday paradox, having a few thousands of changesets will
have a pretty high (statistically) probability of collision, if we only
use 48 bits from the hash: <http://en.wikipedia.org/wiki/Birthday_problem>.

I know internally mercurial uses 160 bits (for instance, in tags), but
what it could happen if I do a "hg log" or a "hg pull -r" with a
truncated hash with a collision?.

Does Mercurial recognize the fact and force you to use the 160 bits in
that case?.

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