Documentation changes to recursive option for git-diff-tree

Update docs and usages regarding '-r' recursive option for git-diff-tree.
Remove '-r' from common diff options, mention it only for git-diff-tree.
Remove one extraneous use of '-r' with git-diff-files in get-merge.sh.
Sync the synopsis and usage string for git-diff-tree.

Signed-off-by: Chris Shoemaker <c.shoemaker at cox.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>

diff.h

@@ -91,7 +91,6 @@ extern void diffcore_std_no_resolve(struct diff_options *);
 
 #define COMMON_DIFF_OPTIONS_HELP \
 "\ncommon diff options:\n" \
-"  -r            diff recursively (only meaningful in diff-tree)\n" \
 "  -z            output diff-raw with lines terminated with NUL.\n" \
 "  -p            output patch format.\n" \
 "  -u            synonym for -p.\n" \