Rename "stg coalesce" to "stg squash"

"squash" is the word git uses for this operation, so we should use it
too to lessen the confusion. Also, it's easier to spell.

We can do the rename without any effort at backwards compatibility
because "stg coalesce" has never been part of any released version.

Signed-off-by: Karl Hasselström <kha@treskal.com>