Figure out default branch using git, fixes #1443, closes #1727.

CHANGELOG.md

@@ -3,6 +3,7 @@
 ## Unreleased
 
 * ![Enhancement][badge-enhancement] MathJax 3 has been updated to `v3.2.0` (minor version bump). ([#1743][github-1743])
+* ![Enhancement][badge-enhancement] Documenter now tries to detect the development branch using `git` with the old default (`master`) as fallback. If you use `main` as the development branch you shouldn't need to specify `devbranch = "main"` as an argument to deploydocs anymore. ([#1443][github-1443], [#1727][github-1727], [#1751][github-1751])
 
 ## Version `v0.27.10`
 
@@ -860,6 +861,7 @@
 [github-1441]: https://github.com/JuliaDocs/Documenter.jl/pull/1441
 [github-1448]: https://github.com/JuliaDocs/Documenter.jl/pull/1448
 [github-1440]: https://github.com/JuliaDocs/Documenter.jl/pull/1440
+[github-1443]: https://github.com/JuliaDocs/Documenter.jl/issues/1443
 [github-1449]: https://github.com/JuliaDocs/Documenter.jl/issues/1449
 [github-1452]: https://github.com/JuliaDocs/Documenter.jl/pull/1452
 [github-1456]: https://github.com/JuliaDocs/Documenter.jl/pull/1456
@@ -946,7 +948,9 @@
 [github-1706]: https://github.com/JuliaDocs/Documenter.jl/pull/1706
 [github-1709]: https://github.com/JuliaDocs/Documenter.jl/pull/1709
 [github-1716]: https://github.com/JuliaDocs/Documenter.jl/pull/1716
+[github-1727]: https://github.com/JuliaDocs/Documenter.jl/pull/1727
 [github-1743]: https://github.com/JuliaDocs/Documenter.jl/pull/1743
+[github-1751]: https://github.com/JuliaDocs/Documenter.jl/pull/1751
 
 [julia-38079]: https://github.com/JuliaLang/julia/issues/38079
 [julia-39841]: https://github.com/JuliaLang/julia/pull/39841

src/Documenter.jl

@@ -329,7 +329,7 @@ include("deployconfig.jl")
         branch = "gh-pages",
         deps = nothing | <Function>,
         make = nothing | <Function>,
-        devbranch = "master",
+        devbranch = nothing,
         devurl = "dev",
         versions = ["stable" => "v^", "v#.#", devurl => devurl],
         forcepush = false,
@@ -415,7 +415,8 @@ deps = Deps.pip("pygments", "mkdocs")
 executed.
 
 **`devbranch`** is the branch that "tracks" the in-development version of the generated
-documentation. By default this value is set to `"master"`.
+documentation. By default Documenter tries to figure this out using `git`. Can be set
+explicitly as a string (typically `"master"` or `"main"`).
 
 **`devurl`** the folder that in-development version of the docs will be deployed.
 Defaults to `"dev"`.
@@ -491,14 +492,25 @@ function deploydocs(;
         deps   = nothing,
         make   = nothing,
 
-        devbranch = "master",
+        devbranch = nothing,
         devurl = "dev",
         versions = ["stable" => "v^", "v#.#", devurl => devurl],
         forcepush::Bool = false,
         deploy_config = auto_detect_deploy_system(),
         push_preview::Bool = false,
     )
 
+    # Try to figure out default branch (see #1443 and #1727)
+    if devbranch === nothing
+        str = try
+            read(pipeline(ignorestatus(setenv(`git remote show origin`; dir=root)); stderr=devnull), String)
+        catch
+            ""
+        end
+        m = match(r"^\s*HEAD branch:\s*(.*)$"m, str)
+        devbranch = m === nothing ? "master" : String(m[1])
+    end
+
     deploy_decision = deploy_folder(deploy_config;
                                     branch=branch,
                                     branch_previews=branch_previews,