new tool: download all GM images for a given builder, ready for skdiff

gm/rebaseline_server/tests/.gitattributes

Index: gm/rebaseline_server/tests/.gitattributes
diff --git a/gm/rebaseline_server/tests/.gitattributes b/gm/rebaseline_server/tests/.gitattributes
new file mode 100644
index 0000000000000000000000000000000000000000..3606445cad0b8212d5272a401d704f3529c7ef65
--- /dev/null
+++ b/gm/rebaseline_server/tests/.gitattributes
@@ -0,0 +1,2 @@
+# All self-test PNG files are fake (human-readable, diffable plaintext files).

[epoger, 2014/01/22 19:58:10]

From here, you can skip ahead to gm/rebaseline_server/url_or_path.py ; all the
intervening files are new self-test expectations.

+*.png    text diff -binary

[epoger, 2014/01/22 19:58:10]

In spite of this git-attr directive, Rietveld won't show the (human readable)
contents of the test .png files.

But if you look at
https://codereview.chromium.org/download/issue143653006_1.diff , you will see
that each .png file contains a single line like this:
contents of bitmap-64bitMD5/3x3bitmaprect/2054956815327187963.png

I have confirmed that the appropriate files are being copied from
gm/rebaseline_server/tests/inputs/fake-gm-imagefiles to
gm/rebaseline_server/tests/outputs/expected/download_test.DownloadTest.test_fetch
, as directed by
gm/rebaseline_server/tests/inputs/gm-actuals/Test-Android-GalaxyNexus-SGX540-Arm7-Release/actual-results.json
.

For example, as directed by this line:
    "bigblurs_565.png" : [ "bitmap-64bitMD5", 2422083043229439955 ],
gm/rebaseline_server/tests/outputs/expected/download_test.DownloadTest.test_fetch/565/bigblurs.png
reads:
    contents of bitmap-64bitMD5/bigblurs/2422083043229439955.png