Improve the monorepo support

[orhun]

Description

This PR simplifies the following command:

cd packages/some_library
git cliff --include-path "packages/some_library/**/*" --repository "../../"

To:

cd packages/some_library
git cliff

When --include-path is not provided, git-cliff will automatically include the current directory and its subdirectories.

Also, the default configuration file (cliff.toml) is now automatically discovered when running git-cliff in a subdirectory of the repository.

For example, if you run from a subdirectory:

$ cd ratatui/ratatui-crossterm

$ git cliff

 INFO  git_cliff > Using configuration from parent directory: /home/orhun/gh/ratatui/cliff.toml
 INFO  git_cliff > Including changes from the current directory: "ratatui-crossterm"

Motivation and Context

See #969

How Has This Been Tested?

Locally.

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (no code change)
• Refactor (refactoring production code)
• Other

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.
• I have formatted the code with rustfmt.
• I checked the lints with clippy.
• I have added tests to cover my changes.
• All new and existing tests passed.

[orhun]

TODO: add a fixture test for this

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 24 lines in your changes missing coverage. Please review.

Project coverage is 42.80%. Comparing base (e8976fe) to head (85c4e9b).

Files with missing lines	Patch %	Lines
git-cliff/src/lib.rs	0.00%	19 Missing ⚠️
git-cliff-core/src/repo.rs	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #976      +/-   ##
==========================================
- Coverage   43.35%   42.80%   -0.54%     
==========================================
  Files          21       21              
  Lines        1712     1734      +22     
==========================================
  Hits          742      742              
- Misses        970      992      +22     

Flag	Coverage Δ
unit-tests	42.80% <0.00%> (-0.54%)	⬇️

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