bisect-builds.py: Save cache in parent scope

bisect-builds.py: Save cache in parent scope

Using 'cache = ...' always assigns ... to the local scope, while it
should be stored in the parent scope instead. Because of this oversight,
the cache only contained one list of revisions at any time, namely the
list of revisions for the current settings (e.g. "-a linux64" or "-l -a
linux64").
This patch fixed the issue by iterating over the cached dictionary and
copying the keys and values to the variable (in the parent scope).

R=rsesek@chromium.org

Committed: https://crrev.com/1c83605f861223ad3b744c0c6fc93f97d3d2b639
Cr-Commit-Position: refs/heads/master@{#330374}

[Robert Sesek, 2015-05-18 15:56:20]

lgtm

[robwu, 2015-05-18 16:25:45]

The CQ bit was checked by rob@robwu.nl

[commit-bot: I haz the power, 2015-05-18 16:25:56]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1131403003/1

[commit-bot: I haz the power, 2015-05-18 16:34:10]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-18 22:40:20]

Patchset 1 (id:??) landed as
https://crrev.com/1c83605f861223ad3b744c0c6fc93f97d3d2b639
Cr-Commit-Position: refs/heads/master@{#330374}