Finding a commit

[Soham Mehta <soham@box.net>]

Given a SHA1 of a commit from one repository (say x), wondering what is 
a proper way to find out if that commit (change) also exists in a 
different repository (say y).

Because SHA1 can change if a commit is cherry-picked around, I cannot 
just grep for that SHA1 from git-rev-list or git-log on 'y'. I need a 
way to know if a commit with identical changes (as in 'x') is also 
present in 'y'.

I realize that Author and Timestamp do not change when the commit is 
moved (fetched, pushed, pulled, rebased, cherry-picked etc). So my 
current solution relies on grepping for the pair of Author-Timestamp 
from git-log on 'y'.

Wondering if there is more appropriate way of doing this?

Thanks.
-Soham