patchbomb: add a 'patchbomb.intro' option
This option allows the user to control the default behavior for
including an introduction message. This avoids having to tirelessly
skip the intro for people contributing to Mercurial.
The three possibles values are:
- always,
- auto (default, current behavior),
- never.
I was thinking of ("true", "false", "") (empty value being auto) but I ruled it
out as too confusing.
This new config option reuses the pre-existing 'patchbomb' section.
(binary:application/octet-stream)