[PATCH] checkout: most of the time we have good leading directories

When "git checkout" wants to create a path, e.g. a/b/c/d/e, after
seeing if the entire thing already exists (in which case we check if
that is up-to-date and do not bother to check it out, or we unlink
and recreate it), we validate that the leading directory path is
without funny symlinks by seeing a/, a/b/, a/b/c/ and then a/b/c/d/
are all without funny symlinks, by calling has_dirs_only_path() in
this order.

When we are checking out many files (imagine: initial checkout),
however, it is likely that an earlier checkout would have already
made sure that the leading directory a/b/c/d/ is in good order; by
first checking the whole path a/b/c/d/ first, we can often bypass
calls to has_dirs_only_path() for leading part.

This cuts down the number of calls to has_dirs_only_path() for
checking out Linux kernel sources afresh from 190k down to 98k.

Signed-off-by: Junio C Hamano <redacted>
---

 * Just a random experimental change I was playing with today,
   looking for low hanging fruits before having to thread the entire
   checkout codepath.

 entry.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/entry.c b/entry.c
index 7b7aa81..e2c0ac6 100644
--- a/entry.c
+++ b/entry.c

@@ -6,9 +6,17 @@
 static void create_directories(const char *path, int path_len,
 			       const struct checkout *state)
 {
-	char *buf = xmalloc(path_len + 1);
-	int len = 0;
+	char *buf;
+	int len;
+
+	for (len = path_len - 1; 0 <= len; len--)
+		if (path[len] == '/')
+			break;
+	if (has_dirs_only_path(path, len, state->base_dir_len))
+		return; /* ok, we have the whole leading directory */
 
+	buf = xmalloc(path_len + 1);
+	len = 0;
 	while (len < path_len) {
 		do {
 			buf[len] = path[len];