Hooks: standardize the release/changes line

[lbartoletti]

Summary

Today, there is no such standardization, and we have something like:

changelog (or changes, etc): url

or

changelog (or changes, etc):
  url

[thierry]

I use to write it on a single line when the total length is < 73 chars, and with a second line when the URL is long.

[tcberner]

.hooks/prepare-commit-msg

82

^it might be slitghly confusing with the Relnotes tag src uses -- how about Changelog:?