Syntax for specifying a command in git config

Hi all,

Certain git config parameters (notably sendemail.smtpServer) expect an 
absolute path in order to use a command. If the value begins with a / 
git interprets it as a command; otherwise, it's interpreted it as a 
server hostname.

Using an absolute path is not very portable, however. If I want to have 
a common shared config file across multiple machines, each of which uses 
a command for smtpServer (e.g. msmtp) but installed at different 
locations, I have to modify the config file on each machine to update 
the absolute path.

Obviously this is not a *huge* deal, but it is a minor annoyance. I'm 
wondering if it would be possible to add a new syntax to config options 
like this. For example,

     [sendemail]
             smtpServer = !msmtp

The leading ! character says "the following value is a command that 
should be looked up in PATH". This is congruent with alias definitions, 
where the leading ! means to run the following value as a command 
directly instead of as a git subcommand.

This also is unambiguous since server hostnames will never begin with a 
! character.

I would be happy to attempt a patch at this change if it's something the 
git maintainers are open to. And if there are any other options that 
this would make sense for, I can add those too.

Thanks,

Greg