add sample hooks that allow cloning over extant files

Cloning a repo over an existing set of files would usually
cause a merge conflict that aborts the process and requires
manual intervention. This pair of hooks handles that case
by manually moving the extant objects out of the way,
completing the checktout, then restoring them to their
original places. The resulting state is a set of unstaged
local changes.

doc/sample_hooks/post-merge-unclobber

+#!/bin/sh
+
+# This finds objects that the pre-merge script moved out of the way to
+# avoid conflicts when running git clone and moves them back to their
+# original places. The result is that the git repository gets checked out
+# and the extant objects end up back in the working directory. Git now
+# sees these as un-staged changes to the working branch and you can deal
+# with them by adding them or reverting.
+
+find -name '*.vcsh-unclobber' -execdir rename .vcsh-unclobber '' {} \;

doc/sample_hooks/pre-merge-unclobber

+#!/bin/sh
+
+# This code does amost exactly what the native VCSH sanity checking code
+# does except that on finding a potential merge conflict, it moves the 
+# extant object out of the way temporarily. The merge then happens cleanly
+# as far as git knows, and a post-merge hook can figure out what to do with
+# the extant versions of the objects.
+
+for object in $(git ls-tree -r origin/master | awk '{print $4}'); do
+	[ -e "$object" ] && mv "$object" "$object.vcsh-unclobber" 
+done