feat: update git command use to determine branch name

lib/pact/provider_verifier/git.rb

@@ -5,12 +5,60 @@
 module Pact
   module ProviderVerifier
     module Git
-      COMMAND = 'git rev-parse --abbrev-ref HEAD'
+      COMMAND = 'git branch --remote --verbose --no-abbrev --contains'.freeze
+      BRANCH_ENV_VAR_names = %w{BUILDKITE_BRANCH CIRCLE_BRANCH TRAVIS_BRANCH GIT_BRANCH GIT_LOCAL_BRANCH APPVEYOR_REPO_BRANCH CI_COMMIT_REF_NAME}.freeze
 
       def self.branch
-        `#{COMMAND}`.strip
-      rescue StandardError => e
-        raise Pact::ProviderVerifier::Error, "Could not determine current git branch using command `#{COMMAND}`. #{e.class} #{e.message}"
+        find_branch_from_env_vars || branch_from_git_command
+      end
+
+      # private
+
+      def self.find_branch_from_env_vars
+        BRANCH_ENV_VAR_names.collect { |env_var_name| branch_from_env_var(env_var_name) }.compact.first
+      end
+
+      def self.branch_from_env_var(env_var_name)
+        val = ENV[env_var_name]
+        if val && val.strip.size > 0
+          val
+        else
+          nil
+        end
+      end
+
+      def self.branch_from_git_command
+        branch_names = nil
+        begin
+          branch_names = execute_git_command
+            .split("\n")
+            .collect(&:strip)
+            .reject(&:empty?)
+            .collect(&:split)
+            .collect(&:first)
+            .collect{ |line| line.gsub(/^origin\//, '') }
+            .reject{ |line| line == "HEAD" }
+
+        rescue StandardError => e
+          raise Pact::ProviderVerifier::Error, "Could not determine current git branch using command `#{COMMAND}`. #{e.class} #{e.message}"
+        end
+
+        validate_branch_names(branch_names)
+        branch_names[0]
+      end
+
+      def self.validate_branch_names(branch_names)
+        if branch_names.size == 0
+          raise Pact::ProviderVerifier::Error, "Command `#{COMMAND}` didn't return anything that could be identified as the current branch."
+        end
+
+        if branch_names.size > 1
+          raise Pact::ProviderVerifier::Error, "Command `#{COMMAND}` returned multiple branches: #{branch_names.join(", ")}. You will need to get the branch name another way."
+        end
+      end
+
+      def self.execute_git_command
+        `#{COMMAND}`
       end
     end
   end