histedit: add experimental config for using the first word of the commit
This allows users to start a commit with "verb! ..." so that when this is
opened in histedit, the default action will be "verb". For example, "roll! foo"
will default to the action "roll". Currently, we'll allow any known verb to be
used but this is experimental.
$ rm -rf a
$ hg init a
$ cd a
$ echo a > a
$ hg ci -Am0
adding a
$ hg tag t1 # 1
$ hg tag --remove t1 # 2
$ hg co 1
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg tag -f -r0 t1
$ hg tags
tip 3:a49829c4fc11
t1 0:f7b1eb17ad24
$ cd ..