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>
diff --git a/Documentation/git-push.txt b/Documentation/git-push.txt
index f7dfe48..a5c6f7d 100644
--- a/Documentation/git-push.txt
+++ b/Documentation/git-push.txt
@@ -120,7 +120,7 @@
 --follow-tags::
 	Push all the refs that would be pushed without this option,
 	and also push annotated tags in `refs/tags` that are missing
-	from the remote but are pointing at committish that are
+	from the remote but are pointing at commit-ish that are
 	reachable from the refs being pushed.
 
 --receive-pack=<git-receive-pack>::