In [1], improved RevWalk.getMergedInto() is introduced to avoid repeated
work while performing RevWalk.isMergedInto() on many refs. Modify
findBranchesReachableFrom() to use it.
[1] I65de9873dce67af9c415d1d236bf52d31b67e8fe
Change-Id: I81d615241638d4093df64b449637af601843a5ed
Signed-off-by: Adithya Chakilam <quic_achakila@quicinc.com>