diff: Introduce --diff-algorithm command line option

Since command line options have higher priority than config file
variables and taking previous commit into account, we need a way
how to specify myers algorithm on command line. However,
inventing `--myers` is not the right answer. We need far more
general option, and that is `--diff-algorithm`.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.h

@@ -333,6 +333,8 @@ extern struct userdiff_driver *get_textconv(struct diff_filespec *one);
 
 extern int parse_rename_score(const char **cp_p);
 
+extern long parse_algorithm_value(const char *value);
+
 extern int print_stat_summary(FILE *fp, int files,
 			      int insertions, int deletions);
 extern void setup_diff_pager(struct diff_options *);