builtin-apply.c: pay attention to -p<n> when determining the name

The patch structure has def_name component that is used to validate the
sanity of a "diff --git" patch by checking pathnames that appear on the
patch header lines for consistency.  The git_header_name() function is
used to compute this out of "diff --git a/... b/..." line, but the code
always stripped one level of prefix (i.e. "a/" and "b/"), without paying
attention to -p<n> option.  Code in find_name() function that parses other
lines in the patch header (e.g. "--- a/..." and "+++ b/..." lines) however
did strip the correct number of leading paths prefixes, and the sanity
check between these computed values failed.

Teach git_header_name() to honor -p<n> option like find_name() function
does.

Found and reported by Steven J. Murdoch who also wrote tests.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-apply.c

diff --git a/builtin-apply.c b/builtin-apply.c
index f667368..36e2f9d 100644
--- a/builtin-apply.c
+++ b/builtin-apply.c
@@ -823,12 +823,13 @@
 
 static const char *stop_at_slash(const char *line, int llen)
 {
+	int nslash = p_value;
 	int i;
 
 	for (i = 0; i < llen; i++) {
 		int ch = line[i];
-		if (ch == '/')
-			return line + i;
+		if (ch == '/' && --nslash <= 0)
+			return &line[i];
 	}
 	return NULL;
 }

t/t4128-apply-root.sh

diff --git a/t/t4128-apply-root.sh b/t/t4128-apply-root.sh
index 8f6aea4..6cc741a 100755
--- a/t/t4128-apply-root.sh
+++ b/t/t4128-apply-root.sh
@@ -58,6 +58,23 @@
 '
 
 cat > patch << EOF
+diff --git a/c/newfile2 b/c/newfile2
+new file mode 100644
+index 0000000..d95f3ad
+--- /dev/null
++++ b/c/newfile2
+@@ -0,0 +1 @@
++content
+EOF
+
+test_expect_success 'apply --directory -p (new file)' '
+	git reset --hard initial &&
+	git apply -p2 --directory=some/sub/dir/ --index patch &&
+	test content = $(git show :some/sub/dir/newfile2) &&
+	test content = $(cat some/sub/dir/newfile2)
+'
+
+cat > patch << EOF
 diff --git a/delfile b/delfile
 deleted file mode 100644
 index d95f3ad..0000000