add hooks to enable auto handing of merge conflicts

These hooks before and after the actual checkout process
will enable a pair of hook scripts to automatically handle
conflicts and potentially allow the checkout of repositories
over existing file sets

doc/hooks

@@ -14,6 +14,12 @@ Available hooks are:
 * post-enter
 * pre-init
 * post-init
+* pre-merge
+    Use this hook to detect and handle merge conflicts before vcsh's native code
+	finds and errors on them. This is useful for allowing clones on top of existing
+	files.
+* post-merge
+    Use this hook to finish handling any merge conflicts found in the pre-merge hook.
 * pre-pull
 * post-pull
 * pre-push

vcsh

@@ -152,6 +152,7 @@ clone() {
 		exit
 	fi
 	git fetch
+	hook pre-merge
 	git ls-tree -r --name-only origin/master | (while read object; do
 		[ -e "$object" ] &&
 			error "'$object' exists." &&
@@ -161,6 +162,7 @@ clone() {
 		fatal "will stop after fetching and not try to merge!
   Once this situation has been resolved, run 'vcsh run $VCSH_REPO_NAME git pull' to finish cloning." 17
 	git merge origin/master
+	hook post-merge
 	hook post-clone
 	retire
 	hook post-clone-retired