[PATCH] Usage-string fixes.

[PATCH] Usage-string fixes.

Usage string fixes to make maintenance easier (only one instance
of a string to update not multiple copies).  I've spotted and
corrected inconsistent usage text in diff-tree while doing this.

Also diff-cache and read-tree usage text have been corrected to
match their up-to-date features.  Earlier, neither "--cached"
form of diff-cache nor "-m single-merge" form of read-tree were
described.

Signed-off-by: Junio C Hamano <junkio@cox.net>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>