filter-branch: skip commits present on --state-branch

The commits in state:filter.map have already been processed, so don't
filter them again. This makes incremental git filter-branch much faster.

Also add tests for --state-branch option.

Signed-off-by: Michael Barabanov <michael.barabanov@gmail.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-filter-branch.sh

@@ -360,6 +360,7 @@ while read commit parents; do
 	git_filter_branch__commit_count=$(($git_filter_branch__commit_count+1))
 
 	report_progress
+	test -f "$workdir"/../map/$commit && continue
 
 	case "$filter_subdir" in
 	"")

t/t7003-filter-branch.sh

@@ -107,6 +107,21 @@ test_expect_success 'test that the directory was renamed' '
 	test dir/D = "$(cat diroh/D.t)"
 '
 
+V=$(git rev-parse HEAD)
+
+test_expect_success 'populate --state-branch' '
+	git filter-branch --state-branch state -f --tree-filter "touch file || :" HEAD
+'
+
+W=$(git rev-parse HEAD)
+
+test_expect_success 'using --state-branch to skip already rewritten commits' '
+	test_when_finished git reset --hard $V &&
+	git reset --hard $V &&
+	git filter-branch --state-branch state -f --tree-filter "touch file || :" HEAD &&
+	test_cmp_rev $W HEAD
+'
+
 git tag oldD HEAD~4
 test_expect_success 'rewrite one branch, keeping a side branch' '
 	git branch modD oldD &&