Replace custom encoder `Constants#encode` by JDK implementation

Using the implementation provided in the JDK since Java 1.6 by
`String#getBytes(Charset)` reduces JGit maintenance effort and improves
performance.

The method Constants#encode was implemented when JGit still used Java
1.5. See [1].

Kudos to Marcin for proposing to use this improvement in RefWriter [2].
I think it should be used generally.

[1] https://repo.or.cz/jgit.git?a=commit;h=bfa3da225f198b19061158499b1135aff07d85b3
[2] https://eclipse.gerrithub.io/c/eclipse-jgit/jgit/+/1195180



Also-By: Marcin Czech <maczech@gmail.com>
Change-Id: I361ed6286b98351a315b8a8ffc3cb845831d35b2
(cherry picked from commit e5d28989)