feat(command): add GitLink

[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: 49 lines in your changes are missing coverage. Please review.

Comparison is base (e605210) 81.31% compared to head (b581511) 78.57%.

Files	Patch %	Lines
lua/gitlinker.lua	42.68%	47 Missing ⚠️
lua/gitlinker/linker.lua	0.00%	1 Missing ⚠️
lua/gitlinker/utils.lua	96.42%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #120      +/-   ##
==========================================
- Coverage   81.31%   78.57%   -2.75%     
==========================================
  Files          12       12              
  Lines         808      840      +32     
==========================================
+ Hits          657      660       +3     
- Misses        151      180      +29     

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