Add more descriptive error message when create branch with invalid name

Author: dbalexandre

app/services/create_branch_service.rb

@@ -4,7 +4,7 @@ class CreateBranchService < BaseService
   def execute(branch_name, ref)
     valid_branch = Gitlab::GitRefValidator.validate(branch_name)
     if valid_branch == false
-      return error('Branch name invalid')
+      return error("Branch name can't contains space, '~', '^', ':', '?', '*', '[', '\', '..', '@{', and consecutive slashes, start with '/' or '.' or end in '/' or '.' or '.lock'")
     end
 
     repository = project.repository

app/views/projects/branches/new.html.haml

@@ -14,6 +14,7 @@
     = label_tag :branch_name, nil, class: 'control-label'
     .col-sm-10
       = text_field_tag :branch_name, params[:branch_name], required: true, tabindex: 1, autofocus: true, class: 'form-control'
+      .help-block Can't contains space, '~', '^', ':', '?', '*', '[', '\', '..', '@{', and consecutive slashes, start with '/' or '.' or end in '/' or '.' or '.lock'
   .form-group
     = label_tag :ref, 'Create from', class: 'control-label'
     .col-sm-10

features/steps/project/commits/branches.rb

@@ -61,7 +61,7 @@ class Spinach::Features::ProjectCommitsBranches < Spinach::FeatureSteps
   end
 
   step 'I should see new an error that branch is invalid' do
-    expect(page).to have_content 'Branch name invalid'
+    expect(page).to have_content "Branch name can't contains space, '~', '^', ':', '?', '*', '[', '\', '..', '@{', and consecutive slashes, start with '/' or '.' or end in '/' or '.' or '.lock'"
   end
 
   step 'I should see new an error that ref is invalid' do