So there is only one build command shared by all `git imerge`s (yes, believe it or not, I have had multiple imerges going on in parallel in a single repo!).  And yet by default the value is not carried over from one completed imerge to a newly-started one, but nor is the old value deleted when a new imerge is finished.  It is a bit inconsistent.

It seem to me that there are two possible self-consistent policies:

* Each imerge has its own test command, maybe stored in `imerge.<name>.testcommand`.  Possibly there is also a default value for when the user doesn't specify a command explicitly, maybe stored under `imerge.defaulttestcommand`.  When an imerge is finished, its `imerge.<name>.testcommand` setting should be erased.

* All imerges use the same command, all the time.  Then it could be stored in `imerge.testcommand`, but then it should be carried over from one run to the next (i.e., if the user starts `git imerge` without setting a new command, then the old command should continue to be used).

I prefer the former.