calcurse-caldav: Add hook support

Introduce pre-sync and post-sync hooks which need to be located under
~/.calcurse/caldav/hooks/ and are executed before/after synchronization
with a CalDAV server.

Also, add an example post-sync hook and change the example post-save
hook such that it does not create tiny commits during synchronization.

Signed-off-by: Lukas Fleischer <lfleischer@calcurse.org>

contrib/caldav/hooks/post-sync

@@ -0,0 +1,12 @@
+#!/bin/sh
+#
+# Copy this file into ~/.calcurse/caldav/hooks/ and mark it executable to
+# automatically commit changes after syncing using Git.
+
+cd "$HOME"/.calcurse/
+
+git add *
+if ! git diff-index --quiet --cached HEAD; then
+	git commit -m "Automatic commit by the post-sync hook"
+fi
+