Add Default Pull Request Title

[eloyekunle]

Set default PR title to commit summary if there's a single commit on the head branch, else set it to the head branch name
Fixes #3582.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@c2dcb35). Click here to learn what that means.
The diff coverage is 81.25%.

@@            Coverage Diff            @@
##             master    #5735   +/-   ##
=========================================
  Coverage          ?   37.76%           
=========================================
  Files             ?      325           
  Lines             ?    47695           
  Branches          ?        0           
=========================================
  Hits              ?    18014           
  Misses            ?    27085           
  Partials          ?     2596

Impacted Files	Coverage Δ
routers/repo/pull.go	34.48% <81.25%> (ø)

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[zeripath]

@eloyekunle Could you just check that if users puts things like &amp; <script>console.log('oh dear')</script> etc in either the summary or the message that these are escaped with your patch. I can't run the code myself to check it at the moment.

[eloyekunle]

@zeripath I've checked and it seems to escape fine.