AdvertisedRequestValidator: fix WantNotValidException caused by race

AdvertisedRequestValidator: fix WantNotValidException caused by race

Fetch with protocol V2 failed under the following conditions
- fetch uses bidirectional protocol (git, ssh) which uses a shortcut
  to determine invalid wants
- not all wants are advertised
- race condition: wanted ref is updated during fetch by another thread
  after the thread serving upload-pack determined wants and before it
  checks not advertised wants

Fix this by calling
`new ReachableCommitRequestValidator().checkWants(up, wants)`
instead of throwing WantNotValidException in [1]
if this race happened in the same way like it's done for unidirectional
protocols (http) [2].

[1] https://github.com/eclipse-jgit/jgit/blob/stable-6.10/org.eclipse.jgit/src/org/eclipse/jgit/transport/UploadPack.java#L2002
[2] https://github.com/eclipse-jgit/jgit/blob/stable-6.10/org.eclipse.jgit/src/org/eclipse/jgit/transport/UploadPack.java#L2000

Bug: jgit-48
Change-Id: I32f28502923815dc49781aab5d810c9afbe7e7e6