feat: option to skip merge commits format check

[zlodes]

Why?

We faced faced that using this pre-commit hook brokes developer's user experience while merging branches manually.

So, there are a couple of use-cases:

Use cases

UC 1: Merging a x.x (release) branch to dev branch using git cli:

git checkout dev
git merge `2.x` -> `:x` in vim

Expected:

Branch is merged, git is clean

Actual:

Pre-commit validation error in console, git merge still in progress

UC 2: Merging a x.x (release) branch to dev branch using IDE (any JetBrains IDE)

Initiate a merge using IDE

Expected:

Branch is merged, git is clean

Actual:

Pre-commit validation error in a small popup, git merge still in progress, developer doesn't even understand what happened

What this PR does?

It adds a new option --skip-merge-commits which allows to skip merge commits under format validation.

Merge commit has commit message prefixed with Merge branch (just made a research in https://github.com/git/git repo).

References

#39
#84

[thekaveman]

Hi @zlodes. Thanks for the PR -- as indicated elsewhere (e.g. in the linked issues) this is not something we are interested in supporting.

There are a number of workarounds for handling merge commits, see this comment for some ideas: #76 (comment)

[zlodes]

@thekaveman sorry, but all the "workarounds" are like crutches, if you use git flow / github flow, you should understand.
Please check the use cases.

The PR doesn't break backward compatibility, tests added, readme updated within the new option.

[egeres]

Finally 😭