Add the ability to prefix something to the pathname to "checkout-cache.c"

Add the ability to prefix something to the pathname to "checkout-cache.c"

This basically makes it trivial to use checkout-cache as a "export as
tree" function. Just read the desired tree into the index, and do a

	checkout-cache --prefix=export-dir/ -a

and checkout-cache will "export" the cache into the specified directory.

NOTE! The final "/" is important. The exported name is literally just
prefixed with the specified string, so you can also do something like

	checkout-cache --prefix=.merged- Makefile

to check out the currently cached copy of "Makefile" into the file
".merged-Makefile".