Grammar tweak in pull request message template

"The first block of the text"
becomes:
"The first block of text"

The definite article was not needed here.
Fix git commit summary in pull request message template

Use "origin/<base>" and "origin/<head>" for git commit comparison and
summary in the pull request template instead of the local branch
equivalents, since the local branches could be out of date with the
remote tracking branches.
Fix normalization of commit summary in pull request test

https://travis-ci.org/github/hub/builds/38441514
Merge pull request #645 from github/base-head-upstream

Fix git commit summary in pull request message template