Perform a single Git fetch when building source distributions (#499)

## Summary

We need to pass in the distribution with the "precise" URL to avoid
refetching.

## Test Plan

Ran `cargo run -p puffin-cli -- pip-compile requirements.in --verbose`
with `flask @ git+https://github.com/pallets/flask.git` and verified
that we only checked out Flask once.

crates/puffin-git/src/source.rs

@@ -128,6 +128,7 @@ impl Fetch {
     pub fn path(&self) -> &Path {
         &self.path
     }
+
     pub fn into_git(self) -> GitUrl {
         self.git
     }