git-diff: do not turn off -p/-M with any diff options.

When the user gives a diff option (e.g. --cached) to "git diff",
we turned off the built-in default option -p, which is usually
not what user wants to see.

This commit makes lack of --name-status, --name-only nor -r to
add -p, and lack of -B*, -C* nor -M* to add -M to the flags
given to the underlying diff.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

git-diff.sh

@@ -7,8 +7,6 @@ rev=$(git-rev-parse --revs-only --no-flags --sq "$@") || exit
 flags=$(git-rev-parse --no-revs --flags --sq "$@")
 files=$(git-rev-parse --no-revs --no-flags --sq "$@")
 
-: ${flags:="'-M' '-p'"}
-
 # I often say 'git diff --cached -p' and get scolded by git-diff-files, but
 # obviously I mean 'git diff --cached -p HEAD' in that case.
 case "$rev" in
@@ -20,6 +18,21 @@ case "$rev" in
 	esac
 esac
 
+# If we do not have --name-status, --name-only nor -r, default to -p.
+# If we do not have -B nor -C, default to -M.
+case " $flags " in
+*" '--name-status' "* | *" '--name-only' "* | *" '-r' "* )
+	;;
+*)
+	flags="$flags'-p' " ;;
+esac
+case " $flags " in
+*" '-"[BCM]* | *" '--find-copies-harder' "*)
+	;; # something like -M50.
+*)
+	flags="$flags'-M' " ;;
+esac
+
 case "$rev" in
 ?*' '?*' '?*)
 	echo >&2 "I don't understand"