fix: Follow Bash best practices in entrypoint

[Kurt-von-Laven]

Draw inspiration from Shellcheck.

• Simplify command-line construction by building up arrays rather than passing empty strings via unquoted variables (c.f., SC2206).
• Prefer [[ ]] to [ ] to prevent globbing and word splitting in tests.
• Quote Bash variables elsewhere to prevent globbing and word splitting (c.f., SC2086).
• Reorder code to start with set directive, fail fast, and colocate related sections.
• Use consistent style recommended by shfmt for if/then/else blocks.
• Use UPPER_CASE consistently for variable names.
• Use single quotes consistently for string literals.
• Use curly braces to delimit variables from surrounding string literals but not in isolation.

Fixes #28.

[woile]

We are getting an error: https://github.com/commitizen-tools/commitizen-action/runs/6383601365?check_suite_focus=true#step:4:70

[Kurt-von-Laven]

Yikes; thanks for the heads up. I've opened #35 to address this matter.