Produce better error messages for non-binary mix git deps refspecs. (#13088)

When using git dependencies, a branch/ref/tag specifier
is passed verbatim to `System.cmd/3`. This can lead to
intimidating error messages when they are not provided
as a binary (for instance, an atom like `tag: :stable`):

```
** (ArgumentError) all arguments for System.cmd/3 must be binaries
    (elixir 1.15.6) lib/system.ex:1083: System.cmd/3
    (mix 1.15.6) lib/mix/scm/git.ex:287: Mix.SCM.Git.git!/2
```

This PR adds a check during git opts verification time to provide
better feedback.

Author: christhekeele

lib/mix/lib/mix/scm/git.ex

@@ -201,18 +201,24 @@ defmodule Mix.SCM.Git do
   ## Helpers
 
   defp validate_git_options(opts) do
-    err =
-      "You should specify only one of branch, ref or tag, and only once. " <>
-        "Error on Git dependency: #{redact_uri(opts[:git])}"
+    case Keyword.take(opts, [:branch, :ref, :tag]) do
+      [] ->
+        opts
 
-    validate_single_uniq(opts, [:branch, :ref, :tag], err)
-  end
+      [{_refspec, value}] when is_binary(value) ->
+        opts
 
-  defp validate_single_uniq(opts, take, error) do
-    case Keyword.take(opts, take) do
-      [] -> opts
-      [_] -> opts
-      _ -> Mix.raise(error)
+      [{refspec, value}] ->
+        Mix.raise(
+          "A dependency's #{refspec} must be a string, got: #{inspect(value)}. " <>
+            "Error on Git dependency: #{redact_uri(opts[:git])}"
+        )
+
+      _ ->
+        Mix.raise(
+          "You should specify only one of branch, ref or tag, and only once. " <>
+            "Error on Git dependency: #{redact_uri(opts[:git])}"
+        )
     end
   end
 

lib/mix/test/mix/scm/git_test.exs

@@ -50,7 +50,7 @@ defmodule Mix.SCM.GitTest do
     end
   end
 
-  test "raises about conflicting Git checkout options" do
+  test "raises about conflicting Git refspec options" do
     assert_raise Mix.Error, ~r/You should specify only one of branch, ref or tag/, fn ->
       Mix.SCM.Git.accepts_options(nil, git: "/repo", branch: "main", tag: "0.1.0")
     end
@@ -60,6 +60,20 @@ defmodule Mix.SCM.GitTest do
     end
   end
 
+  test "raises about non-binary Git refspec options" do
+    assert_raise Mix.Error, ~r/A dependency's branch must be a string/, fn ->
+      Mix.SCM.Git.accepts_options(nil, git: "/repo", branch: :main)
+    end
+
+    assert_raise Mix.Error, ~r/A dependency's tag must be a string/, fn ->
+      Mix.SCM.Git.accepts_options(nil, git: "/repo", tag: :stable)
+    end
+
+    assert_raise Mix.Error, ~r/A dependency's ref must be a string/, fn ->
+      Mix.SCM.Git.accepts_options(nil, git: "/repo", ref: :abcdef0123456789)
+    end
+  end
+
   defp lock(opts \\ []) do
     [lock: {:git, "/repo", "abcdef0123456789", opts}]
   end