rebase: use hard-coded template for one-line commit description
This is to prepare for making making the one-line summary
customizable. The template ended up pretty complicated because of the
conditional output of "(<bookmarks etc>)". Maybe we can simplify the
template later.
Differential Revision: https://phab.mercurial-scm.org/D9250
https://bz.mercurial-scm.org/1089
$ hg init
$ mkdir a
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ hg ci -m m a
$ mkdir a b
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ cd b
Relative delete:
$ hg ci -m m ../a
$ cd ..