performance note sha1 vs md5 in python

[Martin Pool]

Those are interesting rules-of-thumb to keep in mind.

If we're going to use md5 then we could even use md4 and be faster
still, though that would require providing our own C implementation.

There are slight advantages to SHA1 in: consistency, not answering
questions about why we chose it or whether it's a problem, and having
security even for spaces where it's not expected.

If we could do a 64MB commit or pull in less than 2.5s then changing
from sha1 to md5 would be a 10% overall improvement, which would be
noticeable.