revisions.txt: document more special refs

Some special refs, namely HEAD, FETCH_HEAD, ORIG_HEAD, MERGE_HEAD and
CHERRY_PICK_HEAD, are mentioned and described in 'gitrevisions', but some
others, namely REBASE_HEAD, REVERT_HEAD, and BISECT_HEAD, are not.

Add a small description of these special refs.

Signed-off-by: Philippe Blain <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: phil-blain

Documentation/revisions.txt

@@ -32,8 +32,8 @@ characters and to avoid word splitting.
   first match in the following rules:
 +
   . If '$GIT_DIR/<refname>' exists, that is what you mean (this is usually
-    useful only for `HEAD`, `FETCH_HEAD`, `ORIG_HEAD`, `MERGE_HEAD`
-    and `CHERRY_PICK_HEAD`);
+    useful only for `HEAD`, `FETCH_HEAD`, `ORIG_HEAD`, `MERGE_HEAD`,
+    `REBASE_HEAD`, `REVERT_HEAD`, `CHERRY_PICK_HEAD` and `BISECT_HEAD`);
 
   . otherwise, 'refs/<refname>' if it exists;
 
@@ -59,9 +59,18 @@ characters and to avoid word splitting.
   `MERGE_HEAD`:::
     records the commit(s) which you are merging into your branch when you
     run `git merge`.
+  `REBASE_HEAD`:::
+    during a rebase, records the commit at which the operation is
+    currently stopped, either because of conflicts or an `edit` command in
+    an interactive rebase.
+  `REVERT_HEAD`:::
+    records the commit which you are reverting when you run `git revert`.
   `CHERRY_PICK_HEAD`:::
     records the commit which you are cherry-picking when you run `git
     cherry-pick`.
+  `BISECT_HEAD`:::
+    records the current commit to be tested when you run `git bisect
+    --no-checkout`.
 
 +
 Note that any of the 'refs/*' cases above may come either from