Handle missing git on macOS properly

We need to ignore the default CLT shim for git and find the _real_ path ourselves to properly skip git fetches before git is available. Otherwise, it _seems_ present, but is actually missing and will pop up the Xcode CLT installer dialog and fail to fetch git resources

src/mstrap/platform.cr

@@ -12,7 +12,7 @@ module MStrap
 
     # Indicates whether the host platform has Git installed
     def self.has_git?
-      ENV["MSTRAP_IGNORE_GIT"]? != "true" && has_command?("git")
+      ENV["MSTRAP_IGNORE_GIT"]? != "true" && platform.has_git?
     end
 
     # Indicates whether the host platform has a given command available

src/mstrap/platform/darwin/macos.cr

@@ -3,6 +3,35 @@ module MStrap
     module MacOS
       extend DSL
 
+      XCODE_CLT_GIT_PATH = "/Library/Developer/CommandLineTools/usr/bin/git"
+
+      @@git_path : String = ""
+
+      # :nodoc:
+      def self.has_git?
+        git_path = @@git_path
+        return true if git_path != ""
+
+        git_path = Process.find_executable("git")
+
+        # Ignore the XCode CLT shim trickery!
+        if git_path && git_path == "/usr/bin/git"
+          # Try and look it up with xcrun
+          if has_command?("xcrun")
+            xcrun_git_path = `xcrun -find git 2>/dev/null`.chomp
+            git_path = Process.find_executable(xcrun_git_path) if $?.success?
+          end
+
+          # Fallback to default CLT path
+          git_path ||= Process.find_executable(XCODE_CLT_GIT_PATH)
+        end
+
+        return false if git_path.nil?
+
+        @@git_path = git_path
+        true
+      end
+
       def self.install_packages!(packages : Array(String))
         cmd("brew", ["install"] + packages)
       end

src/mstrap/platform/linux.cr

@@ -118,6 +118,10 @@ module MStrap
       distro_family == DISTRO_UNKNOWN
     end
 
+    def has_git?
+      has_command?("git")
+    end
+
     # :nodoc:
     def platform
       if arch_distro?