hook special remote: Added combined hook program support.

doc/special_remotes/hook.mdwn

@@ -50,7 +50,7 @@ These environment variables are used to communicate with the hook commands:
   into 1024 buckets.
 * `ANNEX_HASH_2` - another hash value, can be used for a second level of hashing
 
-The setting to use in git config for the hook commands are as follows:
+The settings to use in git config for the hook commands are as follows:
 
 * `annex.$hooktype-store-hook` - Command run to store a key in the special remote.
   `ANNEX_FILE` contains the content to be stored.
@@ -68,3 +68,38 @@ The setting to use in git config for the hook commands are as follows:
   if and only if the key has been actively verified to be present in the
   special remote (caching presence information is a very bad idea);
   all other output to stdout will be ignored.
+
+## combined hook program
+
+Rather than setting all of the above hooks, you can write a single
+program that handles everything, and set a single hook to make it be used.
+
+	# git config annex.demo-hook /usr/local/bin/annexdemo
+	# git annex initremote mydemorepo type=hook hooktype=demo encryption=none
+
+The program just needs to look at the `ANNEX_ACTION` environment variable
+to see what it's being asked to do  For example:
+
+[[!format sh """
+#!/bin/sh
+set -e
+case "$ANNEX_ACTION" in
+	store)
+		demo-upload "$ANNEX_HASH_1/$ANNEX_HASH_2/$ANNEX_KEY" < "$ANNEX_FILE"
+	;;
+	retrieve)
+		demo-download "$ANNEX_HASH_1/$ANNEX_HASH_2/$ANNEX_KEY" > "$ANNEX_FILE"
+	;;
+	remove)
+		demo-nuke "$ANNEX_HASH_1/$ANNEX_HASH_2/$ANNEX_KEY"
+	;;
+	checkpresent)
+		if demo-exists "$ANNEX_HASH_1/$ANNEX_HASH_2/$ANNEX_KEY"; then
+			echo "$ANNEX_KEY"
+		fi
+	*)
+		echo "unkown ANNEX_ACTION: $ANNEX_ACTION" >&2
+		exit 1
+	;;
+esac
+"""]]