use more portable find syntax

BSD find (e.g. as found on Mac OSX) has more strict rules for parsing
arguments and needs a path. PWD is not assumed as on GNU find.

doc/sample_hooks/post-merge-unclobber

@@ -7,4 +7,4 @@
 # directory. Git and thus vcsh now see these as un-staged changes to the
 # working branch and you can deal with them as usual.
 
-find -name '*.vcsh-unclobber' -execdir rename .vcsh-unclobber '' {} \;
+find . -name '*.vcsh-unclobber' -execdir rename .vcsh-unclobber '' {} \;