Improve regex for MergeConflicts hook

The previous regex could report a false positive if the literal string
of seven '<' characters appeared in the middle of a line. Even if this
is unlikely, we can easily guard against this by anchoring the regex at
the beginning of the line.

Change-Id: I79ea46fbf80c75f7e7b4672ef457e6f52c6cd120
Reviewed-on: http://gerrit.causes.com/37944
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>

Author: Shane da Silva

lib/overcommit/hook/pre_commit/merge_conflicts.rb

@@ -2,7 +2,7 @@ module Overcommit::Hook::PreCommit
   # Checks for unresolved merge conflicts
   class MergeConflicts < Base
     def run
-      result = execute(%w[grep -IHn <<<<<<<] + applicable_files)
+      result = execute(%w[grep -IHn ^<<<<<<<\s] + applicable_files)
 
       unless result.stdout.empty?
         return :bad, "Merge conflict markers detected:\n#{result.stdout}"

spec/overcommit/hook/pre_commit/merge_conflicts_spec.rb

@@ -20,7 +20,7 @@
   end
 
   context 'when file contains a merge conflict marker' do
-    let(:contents) { "Just\n<<<<<<<some\nconflicting text" }
+    let(:contents) { "Just\n<<<<<<< HEAD:filename.txt\nconflicting text" }
 
     it { should fail_hook }
   end
@@ -30,4 +30,10 @@
 
     it { should pass }
   end
+
+  context "when file contains characters that aren't conflict markers" do
+    let(:contents) { 'Just some <<<<<<< arrows' }
+
+    it { should pass }
+  end
 end