Limit the number of commits shown in MRs

This prevents timeouts when creating a MR with 1000s of commits.

app/views/projects/commits/_commits.html.haml

 - unless defined?(project)
   - project = @project
+- if @commits.size > MergeRequestDiff::COMMITS_SAFE_SIZE
+  - commits = @commits.first(MergeRequestDiff::COMMITS_SAFE_SIZE)
+  - overflow = true
+- else
+  - commits = @commits
+  - overflow = false
 
-- @commits.group_by { |c| c.committed_date.to_date }.sort.reverse.each do |day, commits|
+- commits.group_by { |c| c.committed_date.to_date }.sort.reverse.each do |day, commits|
   .row.commits-row
     .col-md-2.hidden-xs.hidden-sm
       %h5.commits-row-date
@@ -13,3 +19,7 @@
       %ul.bordered-list
         = render commits, project: project
   %hr.lists-separator
+
+- if overflow
+  .alert.alert-warning
+    Not shown: #{@commits.size - MergeRequestDiff::COMMITS_SAFE_SIZE} more commits