Re: [PATCH v3 0/2] Add hostname condition to includeIf

The suggestion for having `git var` list GIT_HOSTNAME gave me an idea: perhaps instead of, or in addition to, a `hostname` condition in the `includeif` code, we could:

 * have an `includeif:env:...` condition that tests an env
   variable against a pattern; and/or
 * use $GIT_HOSTNAME as the variable.

We'd then set `GIT_HOSTNAME` to the gethostname() result *unless* it's already set.

This gives users much more flexibility, because:

 * they can use the hostname and/or arbitrary-env-var condition;
 * they can then *set* GIT_HOSTNAME to the short or full
   hostname at their discretion if the default is not suitable
   for some reason; and of course
 * they can, as noted, use `git var` to find the default setting.

Chris