Git fetcher: Don't compute revCount if it's already specified

We don't care if the user (or more likely the lock file) specifies
an incorrect value for revCount, since it doesn't matter for
security (unlikely content hashes like narHash).

Author: edolstra

src/libfetchers/fetchers.cc

@@ -289,11 +289,6 @@ void Input::checkLocks(Input specified, Input & result)
         if (result.getRev() != prevRev)
             throw Error("'rev' attribute mismatch in input '%s', expected %s", result.to_string(), prevRev->gitRev());
     }
-
-    if (auto prevRevCount = specified.getRevCount()) {
-        if (result.getRevCount() != prevRevCount)
-            throw Error("'revCount' attribute mismatch in input '%s', expected %d", result.to_string(), *prevRevCount);
-    }
 }
 
 std::pair<ref<SourceAccessor>, Input> Input::getAccessor(const Settings & settings, ref<Store> store) const

src/libfetchers/git.cc

@@ -740,8 +740,13 @@ struct GitInputScheme : InputScheme
 
         input.attrs.insert_or_assign("lastModified", getLastModified(settings, repoInfo, repoDir, rev));
 
-        if (!getShallowAttr(input))
-            input.attrs.insert_or_assign("revCount", getRevCount(settings, repoInfo, repoDir, rev));
+        if (!getShallowAttr(input)) {
+            /* Skip revCount computation if it's already supplied by the caller.
+               We don't care if they specify an incorrect value; it doesn't
+               matter for security, unlike narHash. */
+            if (!input.attrs.contains("revCount"))
+                input.attrs.insert_or_assign("revCount", getRevCount(settings, repoInfo, repoDir, rev));
+        }
 
         printTalkative("using revision %s of repo '%s'", rev.gitRev(), repoInfo.locationToArg());