perf(routers): always add 'plain=1' for github

[linrongbin16]

…down like rendering

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.

[linrongbin16]

relate: #142

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (9315320) 75.49% compared to head (6c431e6) 78.81%.

Files	Patch %	Lines
lua/gitlinker/routers.lua	88.88%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #156      +/-   ##
==========================================
+ Coverage   75.49%   78.81%   +3.32%     
==========================================
  Files          11       11              
  Lines         959      963       +4     
==========================================
+ Hits          724      759      +35     
+ Misses        235      204      -31     

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