feat(branch): provide DEFAULT_BRANCH and CURRENT_BRANCH components

[linrongbin16]

Regression Test

Platforms

• windows
• macOS
• linux

Hosts

• Test on github.com.
• Test on gitlab.com.
• Test on bitbucket.org.
• Test on codeberg.org.
• Test on git.samba.org.

Functions

• Use GitLink to copy git link.
• Use GitLink! to open git link in browser.
• Use GitLink blame to copy the /blame git link.
• Use GitLink! blame to open the /blame 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: 19 lines in your changes are missing coverage. Please review.

Comparison is base (3e33ba8) 75.64% compared to head (159ced0) 76.86%.

Files	Patch %	Lines
lua/gitlinker/git.lua	10.52%	17 Missing ⚠️
lua/gitlinker/linker.lua	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #159      +/-   ##
==========================================
+ Coverage   75.64%   76.86%   +1.22%     
==========================================
  Files          11       11              
  Lines         973      990      +17     
==========================================
+ Hits          736      761      +25     
+ Misses        237      229       -8     

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[linrongbin16]

relate: #151