Select/exclude a branch from processing

[devanshshukla99]

By default, sphinx-versioned-docs will try to build all tags and branches present in the git repo.
However, this behaviour can be augmented using the --branch command-line-argument, to select/exclude particular branch(s) and tag(s).

They can be specified in the --branch argument as:

• For selecting a branch:
  Mention the branch/tag name in the CLI arugment like:

        $ sphinx-versioned --branch "main,v2.0"
        $ sphinx-versioned --branch "+main,+v2.0"

Either of the two options above will select main, v2.0 and will only build these.

• For excluding a branch:
  Mention the branch/tag name with - in the CLI argument like:

        $ sphinx-versioned --branch "-main,-v2.0"

The above command will build all available branches and tags except main, v2.0

• For selecting and excluding simultaneously:
  Mention the branch/tag name with - in the CLI argument like:

      $ sphinx-versioned --branch "main,-v2.0"
  

The above command will only build main and will exclude -v2.0 (untouched because the package was only building main).

Some caveats while selecting/excluding branches:

• When selecting some branches, the extension will only build these.

• When excluding some branches, the extension will exclude those and build all other branches/tags present.

• Excluding a branch will always take precedence over selecting a branch.

• Build tests passed

• Codestyle tests passed

• Added test coverage for new features

• Documentation updated to reflect the changes

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Fixes #68

[devanshshukla99]

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