pgsql: Add a --brief option to git_changelog.

Add a --brief option to git_changelog.

In commit c0b050192, Andres introduced the idea of including one-line
commit references in our major release notes. Teach git_changelog to
emit a (lightly adapted) version of that format, so that we don't
have to laboriously add it to the notes after the fact. The default
output isn't changed, since I anticipate still using that for minor
release notes.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/8067c8f86b5f4516ee204a119a750329f7d126ee

Modified Files
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src/tools/git_changelog | 47 +++++++++++++++++++++++++++++++++++++----------
1 file changed, 37 insertions(+), 10 deletions(-)