commit_baseurl is broken in postgres.sgml

Hi Hacker,

I just noticed $SUBJECT:

In doc/src/sgml/postgres.sgml, line 12:

```

<!ENTITY commit_baseurl "https://postgr.es/c/">

```

https://postgr.es/c/ is resolved to https://git.postgresql.org/pg/commitdiff/, but the URL now returns 404.

Looks like Bruce fixed the URL 13 months ago, but it’s broken again. I am not sure if https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff is the correct URL, and anyway I don’t know how to generate a short URL.