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>

contrib/examples/git-merge.sh

@@ -263,7 +263,7 @@ fi
 
 # 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
+# people might have misused the interface and used a 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.