Change the size of the short hash representation

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Change the size of the short hash representation

First note that this number doesn't really matter, as we always check
for ambiguous short hash ids.

Here's the math on collision probability:

>>> import math
>>> def p(f, n): return 1 - (1 / math.exp(n**2/(2*f)))
...
>>> p(2**32, 30000.0)
0.09947179164613551   # with 30000 changesets (BKCVS), we have a 9% chance
>>> p(2**32, 65000.0)
0.38850881217977273   # and with a full import from BK, we'd have a 39% chance
>>> p(2**40, 1e6)
0.36539171908447321   # we'd like to be "safe" for 1M csets, so 40 isn't enough
>>> p(2**48, 1e6)
0.001774780051374103  # But 48 looks good
>>> p(2**48, 1e7)
0.16275260939624481
>>> p(2**48, 5e6)
0.043437281083569146
>>> p(2**48, 2e6)
0.0070802434913129764
>>> p(2**48, 3e6)
0.01586009440574343

manifest hash: 24d9f928a463f46708b0e11fb781d5a241851424
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mercurial/revlog.py

@@ -13,7 +13,7 b' from mercurial import mdiff'
 
 def hex(node): return binascii.hexlify(node)
 def bin(node): return binascii.unhexlify(node)
-def short(node): return hex(node[:4])
+def short(node): return hex(node[:6])
 
 def compress(text):
     if not text: return text