Fix bug in master/buildbot/db/sourcestamps.py

Dustin says:

```
<djmitche> you only get multiple changes attached to a sourcestamp if you have a long enough treeStableTimer that the schedule deals with both changes at the same time
<djmitche> asking "which changes led to this sourcestamp", translated to, say, git terms means "which commits led to this revision"
<djmitche> to which the only logical answer is "all of them"
<djmitche> what we really want is "what are the commits *between* this sourcestamp and that sourcestsamp"
<djmitche> so a sourcestamp will have a single change (the commit that generated it)
<djmitche> and we'll do some tree traversal to answer theq uesetion
<djmitche> but until that happens, it's this kind of crappy "it depends what the scheduler sees" answer :(
```

But with the current implementation, a sourcestamp is created for each change, because "revision" (GIT SHA1 in case of the gitpoller) is used to calculate the ss_hash.
As a consequence, buildbot creates a new sourcestamp for each new changes.