Fix a concurrency issue happening with deep dependencies when the intermediary

Fix a concurrency issue happening with deep dependencies when the intermediary
directory doesn't exist, on fresh checkout and --jobs >1.

It happens in the case where there is 3 dependencies:
a
a/b/c/d
a/b/c/e

'a' is processed first. Then 'a/b/c/d' and 'a/b/c/e' are fired simultaneously.
If both are not present, both svn checkout tries to mkdir b and b/c and a race
condition occurs between their call for isdir() and mkdir().

This works around the issue by creating the intermediary directories ourself
before calling out svn and managing the error ourself.

TBR=dpranke@chromium.org
BUG=
TEST=fresh checkout shouldn't be flaky


Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=106636

[M-A Ruel, 2011-10-20 23:38:23]

It occurs only on fresh checkouts, checking in since it's seriously affecting
our TPMs.

[commit-bot: I haz the power, 2011-10-20 23:38:33]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/maruel@chromium.org/8359018/1

[commit-bot: I haz the power, 2011-10-20 23:44:23]

Change committed as 106636

[Dirk Pranke, 2011-10-21 00:00:45]

lgtm