perf: restructure code & improve unit tests 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: 36 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
lua/gitlinker/git.lua	68.24% <100.00%> (+57.45%)	⬆️
lua/gitlinker/path.lua	93.10% <93.10%> (ø)
lua/gitlinker/range.lua	90.00% <90.00%> (ø)
lua/gitlinker/linker.lua	68.18% <68.18%> (ø)
lua/gitlinker.lua	59.21% <19.04%> (+18.85%)	⬆️

... and 1 file with indirect coverage changes

📢 Thoughts on this report? Let us know!.