blame: accept --need-minimal

Between v1.7.1 and v1.7.2, 582aa00bdf switched the default "diff"
invocation not to use XDF_NEED_MINIMAL, but this breaks "git blame"
rather badly.

Allow the command line option to ask for an extra careful matching.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/blame.c

@@ -2320,6 +2320,7 @@ int cmd_blame(int argc, const char **argv, const char *prefix)
 		OPT_BIT('s', NULL, &output_option, "Suppress author name and timestamp (Default: off)", OUTPUT_NO_AUTHOR),
 		OPT_BIT('e', "show-email", &output_option, "Show author email instead of name (Default: off)", OUTPUT_SHOW_EMAIL),
 		OPT_BIT('w', NULL, &xdl_opts, "Ignore whitespace differences", XDF_IGNORE_WHITESPACE),
+		OPT_BIT(0, "minimal", &xdl_opts, "Spend extra cycles to find better match", XDF_NEED_MINIMAL),
 		OPT_STRING('S', NULL, &revs_file, "file", "Use revisions from <file> instead of calling git-rev-list"),
 		OPT_STRING(0, "contents", &contents_from, "file", "Use <file>'s contents as the final image"),
 		{ OPTION_CALLBACK, 'C', NULL, &opt, "score", "Find line copies within and across files", PARSE_OPT_OPTARG, blame_copy_callback },