Set default branch explicitly

[christopher-dG]

Closes #233

The Git plugin will set the default branch with the following priority:

• The branch keyword to the Git constructor
• The Git setting init.defaultBranch
• "main"

And all the other plugins can use that where they need to refer to the default branch.

This needs the default_branch from #242.

This adds a bit of awkwardness for testing with preexisting Git configurations that set init.defaultBranch to something other than main, since a bunch of tests assume that it'll be unset. I added with_clean_gitconfig to sort of solve that, but the mocking is too slow to use on the reference tests so they're just disabled in that case.

[codecov]

Codecov Report

Merging #243 (2b9e61d) into master (70ba6f7) will decrease coverage by 81.51%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #243       +/-   ##
==========================================
- Coverage   82.18%   0.67%   -81.52%     
==========================================
  Files          19      19               
  Lines         612     594       -18     
==========================================
- Hits          503       4      -499     
- Misses        109     590      +481     

Impacted Files	Coverage Δ
src/plugins/ci.jl	0.00% <ø> (-88.00%)	⬇️
src/plugins/coverage.jl	0.00% <ø> (-90.91%)	⬇️
src/plugins/git.jl	0.00% <ø> (-83.61%)	⬇️
src/plugins/compat_helper.jl	0.00% <0.00%> (-100.00%)	⬇️
src/plugins/readme.jl	0.00% <0.00%> (-92.31%)	⬇️
src/plugins/tagbot.jl	0.00% <0.00%> (-88.89%)	⬇️
src/template.jl	3.17% <0.00%> (-87.38%)	⬇️
src/show.jl	0.00% <0.00%> (-85.72%)	⬇️
src/PkgTemplates.jl	0.00% <0.00%> (-85.72%)	⬇️
src/plugin.jl	0.00% <0.00%> (-83.88%)	⬇️
... and 12 more

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[nickrobinson251]

bump :)

[abelsiqueira]

I would love to see this PR merged soon. Specifically, I'm preparing a tutorial and noticed that devbranch="main" is missing from docs/make.jl. I'm also keeping an eye at JuliaDocs/Documenter.jl#1443.

[christopher-dG]

If anyone has time to take over, they should feel free -- I currently do not, unfortunately.

[abelsiqueira]

I could try to understand and fix this PR, but it is massive, and I don't know the project well enough. So I would probably just get frustrated along the way.
Instead, I could try to just fix the make.jl part. Is that of interest? At least this would push something over the main branch border and might be the kickstart for the rest of the changes.

By the way, my concern here is that I'm using PkgTemplates with mostly default options in my tutorial, which creates a git repo with main branch, but Documenter.jl still uses master, so the default is not consistent.

[timholy]

Superseded by #316, please close (I forgot to put that in my PR OP).