Remove `same-graph` merging in resolver (#6077)
## Summary
This was added in https://github.com/astral-sh/uv/pull/5405 but is now
the cause of an instability in `github_wikidata_bot`. Specifically, on
the initial run, we fork in `pydantic==2.8.2`, via:
```
Requires-Dist: typing-extensions>=4.12.2; python_version >= '3.13'
Requires-Dist: typing-extensions>=4.6.1; python_version < '3.13'
```
In the end, we resolve a single version of `typing-extensions`
(`4.12.2`)... But we don't recognize the two resolutions as the "same
graph", because we propagate the fork markers, and so the "edges" have
different markers on them...
In the second run through, when we have the forks in advance, we don't
split on Pydantic... We just try to solve from the root with the current
forks. This is fundamentally different and I fear it will be the cause
of many instabilities. But removing this graph check fixes the proximate
issue.
I don't really understand why this was added since there was no test
coverage in the PR.
