Before this patch, if you ran more than 1 instance as the same OS user, they'd
use the same config file path and overwrite it and cause post hooks to have
errors due to wrong config being used.
In Darcs, any command can have a post hook (and a pre hook), and the hook
command can be set using a command-line option to the darcs command that you
run. So, in the Vervis SSH server, if we add a --posthook option when running
`darcs apply` to apply remotely received patches, we get a chance to process
the patch data much like in the git post-receive hook.
The setup this patch creates is similar to the git one: It writes a
_darcs/prefs/defaults file to all Darcs repos, and that defaults file sets the
posthook line for `darcs apply`. The posthook line simply executes the actual
hook program written in Haskell.
The current hook program is a one-liner that prints a line to stdout, so every
time you `darcs push` you can tell the hook got executed. The next step is to
implement the actual hook logic, by reading patch data from the environment
variable in which Darcs puts it.
If `darcs init` isn't given a `--repodir`, even if you do specify the
new repository's path, it complains that it can't run inside a
repository, because it's running from a darcs clone of Vervis itself. If
the repo dir is specified using `--repodir` instead, Darcs doesn't
complain.
That's at least the situation with 2.8.5, didn't check other versions.
