filter-branch: return 2 when nothing to rewrite

Using the --state-branch option allows us to perform incremental filtering.
This may lead to having nothing to rewrite in subsequent filtering, so we need
a way to recognize this case.
So, let's exit with 2 instead of 1 when this "error" occurs.

Signed-off-by: Michele Locati <michele@locati.it>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-filter-branch.txt

@@ -222,6 +222,14 @@ this purpose, they are instead rewritten to point at the nearest ancestor that
 was not excluded.
 
 
+EXIT STATUS
+-----------
+
+On success, the exit status is `0`.  If the filter can't find any commits to
+rewrite, the exit status is `2`.  On any other error, the exit status may be
+any other non-zero value.
+
+
 Examples
 --------
 

git-filter-branch.sh

@@ -310,7 +310,7 @@ git rev-list --reverse --topo-order --default HEAD \
 	die "Could not get the commits"
 commits=$(wc -l <../revs | tr -d " ")
 
-test $commits -eq 0 && die "Found nothing to rewrite"
+test $commits -eq 0 && die_with_status 2 "Found nothing to rewrite"
 
 # Rewrite the commits
 report_progress ()