ever-green: determine GIT_DIR correctly again

A recent change in `git rebase` stopped setting `GIT_DIR` when running
the `exec` commands in th todo list. However, the `ever-green.sh` script
relied on it. Let's shell out to Git to determine the Git directory
correctly again.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

ever-green.sh

@@ -205,7 +205,7 @@ nested-rebase)
 	if test -n "$merging"
 	then
 		cat >>"$todo" <<-EOF
-		exec git merge -s ours -m "\$(cat "\$GIT_DIR/merging-rebase-message")" "$merging"
+		exec git merge -s ours -m "\$(cat "\$(git rev-parse --git-dir)/merging-rebase-message")" "$merging"
 		exec git replace --graft HEAD HEAD^
 		exec exit 123 # force re-reading of replacement objects
 		EOF
@@ -817,7 +817,7 @@ then
 	if test -n "$merging"
 	then
 		cat >>"$replace_todo" <<-EOF
-		exec git merge -s ours -m "\$(cat "\$GIT_DIR/merging-rebase-message")" "$current_tip"
+		exec git merge -s ours -m "\$(cat "\$(git rev-parse --git-dir)/merging-rebase-message")" "$current_tip"
 		exec git replace --graft HEAD HEAD^
 		exec exit 123 # force re-reading of replacement objects
 		EOF