perf: improve unit test coverage

[linrongbin16]

Thanks to your contribute, while please finish below tasks:

Regression Test

Platforms

• windows
• macOS
• linux

Tasks

• Use <leader>gl to copy git link.
• Use <leader>gL to open git link in browser.
• Copy git link in a symlink directory of git repo.
• Copy git link in an un-pushed git branch, and receive an expected error.
• Copy git link in a pushed git branch but edited file, and receive a warning says the git link could be wrong.

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
lua/gitlinker/git.lua	74.52% <78.94%> (+17.08%)	⬆️

... and 2 files with indirect coverage changes

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