perf(highlight): allow user custom highlight group 'NvimGitLinkerHighlightTextObject'

[linrongbin16]

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	70.00% <ø> (+8.21%)	⬆️
lua/gitlinker/linker.lua	70.00% <ø> (-20.39%)	⬇️
lua/gitlinker/path.lua	91.66% <ø> (-1.44%)	⬇️
lua/gitlinker.lua	60.24% <80.00%> (-6.43%)	⬇️
lua/gitlinker/highlight.lua	84.00% <80.00%> (+70.66%)	⬆️

... and 2 files with indirect coverage changes

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