* admin/gitmerge.el (gitmerge-missing): Use '--left-only' since we
only want commits from the branch that is to be merged.
(gitmerge-setup-log-buffer): Use the same symmetric range as in
`gitmerge-missing'.
* admin/gitmerge.el (gitmerge-default-branch): Change to
origin/emacs-25.
(gitmerge-missing): Use symmetric difference ('...') between
branch and master so that cherry-picks can be detected.