mob start with ci skip option

[takaiyuk]

Summary

Implement CI skipping function when mob start.
Check out #353 for details.

Implementation (editted)

• Skip CI when mob start
  • CI is skipped by committing empty with message mob start [ci-skip] [ci skip] [skip ci].
• Remove --push-option ci.skip
  • It may by effective only for GitLab at the moment.
  • This requires us to fix test: 5810d58
• Drop the initial empty commit when mob done --squash-wip
  • It is removed by markStartCommitForDropping().
  • It is squashed when mob done or mob done --squash.
  • It is not removed when mob done --no-squash.

[gregorriegler]

Thank you, well done.
I think we can omit the configuration code all together, as we always want to skip the CI.
We made the same decision when we implemented the push options.
Then the change becomes much smaller.
Less code = better, less maintenance.

[gregorriegler]

Feels weird that this constant is in the same place as the other 3 that share a concept. But I don't know how to solve that better now.

[takaiyuk]

To define it as variable instead of constant is one of the options.

[takaiyuk]

Or, introduce new type, something like SquashOption, to clarify the difference:

type SquashOption string

const (
	Squash    = SquashOption("squash")
	NoSquash  = SquashOption("no-squash")
	SquashWip = SquashOption("squash-wip")

	StartCISkipCommitMessage = "mob start [ci-skip] [ci skip] [skip ci]"
)

[gregorriegler]

LGTM, can you please increase the version number to 4.2.0 in

mob/mob.go

Line 29 in 4ac1d3e

versionNumber = "4.1.2"

and add a changelog entry describing what we did?
I think then we can merge.

[takaiyuk]

I did, please fix the changelog message if necessary.

Thanks for discussing and reviewing, @gregorriegler! I am so excited for my PR to be released.