build: Move source map option from git-build to pre-test

Follows-up 0dafebd which moved the source map option from being
always enabled to being only enabled in git-build.

However when a user wants to distribute pre release, they run
'git-build'. But still, they wouldn't want source maps (which
would be just as broken as for a stable release).

Instead move it around into a 'pre-test' task.

Also renamed 'git-build' to 'pre-git-build' and redefined
'git-build' as alias for 'pre-git-build' and 'build'.

This allows users to invoke "grunt git-build" instead of having
to "grunt git-build build".

Change-Id: I15281deb32da7cf32d13d3418532017aaaa03ec1