Add the ability to override parents on RevCommit.

Add the ability to override parents on RevCommit.

This makes RevCommit extensible to allow having different structure of
child-parent relationship. This change is a pre-requsite for having a
FilteredRevCommit that overrides parents from the RevCommit. That then
provides a cheaper way to walk over a subset of RevCommits instead of
an expensive way that applies filters while walking over selected
commits. Useful with Blame which works on a single file and that can be
made performant, if we know all the commits needed by the Blame
algorithm. So Blame algorithm can avoid walking over finding what
commits to blame on.

This change makes parents field on RevCommit private and exposes it
thrrough overrideable methods such as getParents, getParent at index,
getParentCount and setParents. All other files other than RevCommit are
updating the usages of accessing RevCommits parents.

Change-Id: I2d13b001c599cc4ebc92d1ab6e07b07acb3b7fe5