Fixed method to use git log via Popen as recommended, and made output match test (and thus system) expectations.

app/models/repository.rb

@@ -91,7 +91,12 @@ class Repository
     # Limited to 1000 commits for now, could be parameterized?
     args = %W(git log --pretty=%H --max-count 1000 --grep=#{query})
 
-    Gitlab::Popen.popen(args, path_to_repo)
+    git_log_results = Gitlab::Popen.popen(args, path_to_repo)
+
+    # 1. Get result, which is 1-element array
+    # 2. Split on lines
+    # 3. Recreate array, but remove trailing newline characters on each element
+    git_log_results.first.lines.map{ |l| l.chomp }
   end
 
   def find_branch(name)

spec/models/repository_spec.rb

@@ -27,7 +27,7 @@ describe Repository do
   end
 
   describe :find_commits_with_matching_log do
-    subject { repository.find_commits_with_matching_log('submodule').map{|k| k.id} }
+    subject { repository.find_commits_with_matching_log('submodule') }
 
     it { is_expected.to include('5937ac0a7beb003549fc5fd26fc247adbce4a52e') }
     it { is_expected.to include('6f6d7e7ed97bb5f0054f2b1df789b39ca89b6ff9') }