Immediately sends local's heads to the server to check whether the server knows them all.
If it does, we can call getbundle immediately.
Interesting test output changes are:
- added 1 changesets with 0 changes to 1 files (+1 heads)
+ added 1 changesets with 0 changes to 0 files (+1 heads)
-> The new getbundle() actually fixes a bug vs. changegroupsubset() in that it no longer
returns unnecessary files when file revs are reused.
warning: repository is unrelated
+ requesting all changes
-> The new use of common instead of bases correctly indicates that an unrelated pull
gets all changes from the server.