danger: Allow section prefix in commit messages

Signed-off-by: Rémy Coutable <remy@rymai.me>

lib/gitlab/danger/commit_linter.rb

@@ -191,7 +191,7 @@ module Gitlab
       end
 
       def subject_starts_with_lowercase?
-        first_char = subject.sub(/\A\[.+\]\s/, '')[0]
+        first_char = subject.sub(/\A(\[.+\]|\w+:)\s/, '')[0]
         first_char_downcased = first_char.downcase
         return true unless ('a'..'z').cover?(first_char_downcased)
 

spec/lib/gitlab/danger/commit_linter_spec.rb

@@ -190,7 +190,9 @@ RSpec.describe Gitlab::Danger::CommitLinter do
       [
         '[ci skip] A commit message',
         '[Ci skip] A commit message',
-        '[API] A commit message'
+        '[API] A commit message',
+        'api: A commit message',
+        'API: A commit message'
       ].each do |message|
         context "when subject is '#{message}'" do
           let(:commit_message) { message }
@@ -207,6 +209,9 @@ RSpec.describe Gitlab::Danger::CommitLinter do
         '[ci skip]A commit message',
         '[Ci skip]  A commit message',
         '[ci skip] a commit message',
+        'API: a commit message',
+        'API: a commit message',
+        'api: a commit message',
         '! A commit message'
       ].each do |message|
         context "when subject is '#{message}'" do