Optimize upgrade of already-satisfied pinned requirement

Example: after installing six 1.12.0, `pip install -Uv six==1.12.0`
now returns immediately, instead of going to the index to check for a
version that can’t possibly be considered better.

This optimization is most significant when upgrading via a
requirements file with many pinned versions and some non-pinned
versions.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

src/pip/_internal/index.py

@@ -874,10 +874,15 @@ def find_requirement(self, req, upgrade):
         Returns a Link if found,
         Raises DistributionNotFound or BestVersionAlreadyInstalled otherwise
         """
-        hashes = req.hashes(trust_internet=False)
-        best_candidate_result = self.find_best_candidate(
-            req.name, specifier=req.specifier, hashes=hashes,
-        )
+        if req.is_pinned and req.satisfied_by is not None:
+            # Pinned version is already satisfied; no need to check
+            # the index for a better version.
+            best_candidate_result = BestCandidateResult([], [], None)
+        else:
+            hashes = req.hashes(trust_internet=False)
+            best_candidate_result = self.find_best_candidate(
+                req.name, specifier=req.specifier, hashes=hashes,
+            )
         best_candidate = best_candidate_result.best_candidate
 
         installed_version = None    # type: Optional[_BaseVersion]