rebase: improve help text
- add a paragraph about default dest/source changesets
- option help: say "changeset" not "revision"
- option help: explain -b/--base better
- clarify that -a/--abort and -c/--continue are different from the
other options
adding a
tree a0c8bcbbb45c
parent 000000000000
author test 0 0
committer test 0 0
revision 0
branch default
adda