File compile and API errors in JGit

File compile and API errors in JGit

* Photon throws null analysis errors on the repeated invocation of those
previously null checked methods. Extract them to a local variable to
avoid this. (the null analysis is configured in project properties)
* setUseProtocolV2() misses @since tag. Problem was introduced with
332bc611249d21f9b604f2c0207bf0bdfbfc3a78. Might be caused by the long
delay of 2 months from creation to merging.

Change-Id: Ibbb1a1580b604b8e7cd4bf7edc4643e292b6b4a8
Signed-off-by: Michael Keppler <Michael.Keppler@gmx.de>