Change error code "already exists" to "non-fast forward"

The "already exists" error code relies on a change in git.git "next",
which is (as far as I can tell) in no released version of git.
On the other hand, the "non-fast forward" code has been supported
since git v1.7.10
Merge pull request #10 from fingolfin/older-git

Change error code "already exists" to "non-fast forward"
Fix tests for non-fast-forward errors
Only check whether DEBUG_GITIFYHG is set

git has multiple debug env variables, but for all of them,
it only matters if they are set or not -- the exact value
is irrelevant. To avoid surprise, gitifyhg should do the same.
Merge pull request #12 from fingolfin/patch-1

Only check whether DEBUG_GITIFYHG is set