add backout command.
command undoes effect of an earlier commit, commits new changeset
as result.
# basic operation
adding a
changeset 2:b38a34ddfd9f backs out changeset 1:a820f4f40a57
a
# file that was removed is recreated
adding a
adding a
changeset 2:44cd84c7349a backs out changeset 1:76862dcce372
content
# backout of backout is as if nothing happened
removing a
changeset 3:0dd8a0ed5e99 backs out changeset 2:44cd84c7349a
cat: a: No such file or directory
# backout with merge
adding a
changeset 3:6c77ecc28460 backs out changeset 1:314f55b1bf23
merging with changeset 2:b66ea5b77abb
merging a
line 1