RepoProject: read the 'dest-branch' attribute of a project

The manifest spec [1] defines a "dest-branch" attribute. Parse its
value and store it in the RepoProject. Also, create a getter/setter
for dest-branch.

[1] https://gerrit.googlesource.com/git-repo/+/master/docs/manifest-format.md#Element-project

Change-Id: I8ad83b0fec59d2b0967864e4de4fefde4ab971ff
(cherry picked from commit 47fd412a)