Add a class for interacting with git config

This commit adds the GitConfigStore class. This allows us to easily
get and set keys and values in git config.

Tests pass.
Store the test command in git config

This commit uses GitConfigStore to read and write the test command
using git config. The test command is stored in a different place
for each imerge. The key is of the form: imerge.name.testcommand.
The test command is removed when the imerge is finished.

Also fixed the test-build script to use the make_script in the current dir.