Add git-reup and friends

docs/src/git-reparent-branch.txt

+git-reparent-branch(1)
+=====================
+
+NAME
+----
+git-reparent-branch -
+include::_git-reparent-branch_desc.helper.txt[]
+
+SYNOPSIS
+--------
+[verse]
+'git reparent-branch' <new_parent>
+
+DESCRIPTION
+-----------
+
+Change the 'upstream' of the current branch, and then run
+linkgit:git-rebase-update[1] to move the commits in the current branch, as well
+as the commits in all descendant branches, onto the new parent.
+
+`<new_parent>` may be either a local branch, remote branch, OR a tag (such as
+`lkgr`).
+
+This is particularly useful to reparent an independent CL to become dependent on
+another CL, or vice versa. This could happen if you started both on the
+assumption that they were independent, but later realized that this was not the
+case.
+
+
+include::_aliases.txt[]
+
+----
+[alias]
+  git rp = reparent-branch
+----
+
+SEE ALSO
+--------
+linkgit:git-rebase-update[1], linkgit:git-rename-branch[1],
+linkgit:git-new-branch[1], linkgit:git-upstream-diff[1]
+
+include::_footer.txt[]
+
+// vim: ft=asciidoc noexpandtab: