use 'commit-ish' instead of 'committish'

Replace 'committish' in documentation and comments with 'commit-ish'
to match gitglossary(7) and to be consistent with 'tree-ish'.

The only remaining instances of 'committish' are:
  * variable, function, and macro names
  * "(also committish)" in the definition of commit-ish in
    gitglossary[7]

Signed-off-by: Richard Hansen <rhansen@bbn.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/merge.c

@@ -1193,7 +1193,7 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 	 * This could be traditional "merge <msg> HEAD <commit>..."  and
 	 * the way we can tell it is to see if the second token is HEAD,
 	 * but some people might have misused the interface and used a
-	 * committish that is the same as HEAD there instead.
+	 * commit-ish that is the same as HEAD there instead.
 	 * Traditional format never would have "-m" so it is an
 	 * additional safety measure to check for it.
 	 */