Rename rev-parse --abbrev to --short.

The usage of rev-parse to serve as a flag/option parser
for git-whatchanged and other commands have serious limitation
that the flags cannot be something that is supported by
rev-parse itself, and it cannot worked around easily.  Since
this is rarely used "poor-man's describe", rename the option for
now as an easier workaround.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

rev-parse.c

@@ -199,8 +199,8 @@ int main(int argc, char **argv)
 				verify = 1;
 				continue;
 			}
-			if (!strcmp(arg, "--abbrev") ||
-			    !strncmp(arg, "--abbrev=", 9)) {
+			if (!strcmp(arg, "--short") ||
+			    !strncmp(arg, "--short=", 9)) {
 				filter &= ~(DO_FLAGS|DO_NOREV);
 				verify = 1;
 				abbrev = DEFAULT_ABBREV;