Use gclient's git_cache setting in bot_update/api.py

recipe_modules/gclient/example.expected/basic.json

Index: recipe_modules/gclient/example.expected/basic.json
diff --git a/recipe_modules/gclient/example.expected/basic.json b/recipe_modules/gclient/example.expected/basic.json
index 46c74809df03069d97da2ffd51b9e6ee735263e1..04832b971579dcc56c608b4fc60c1e39bb1a20fa 100644
--- a/recipe_modules/gclient/example.expected/basic.json
+++ b/recipe_modules/gclient/example.expected/basic.json
@@ -89,7 +89,7 @@
       "RECIPE_PACKAGE[depot_tools]/gclient.py",
       "config",
       "--spec",
-      "cache_dir = None\nsolutions = [{'deps_file': 'DEPS', 'managed': True, 'name': 'WebKit', 'safesync_url': 'https://blink-status.appspot.com/lkgr', 'url': 'svn://svn.chromium.org/blink/trunk'}]"
+      "cache_dir = ''\nsolutions = [{'deps_file': 'DEPS', 'managed': True, 'name': 'WebKit', 'safesync_url': 'https://blink-status.appspot.com/lkgr', 'url': 'svn://svn.chromium.org/blink/trunk'}]"

[agable, 2016/03/03 17:42:31]

I'm not sure I like cache_dir='' here.

[szager1, 2016/03/03 18:03:02]

This is dictated by recipe_engine.config.Single, which has strict rules about
types.  Here are a couple of ways to work around that, neither of which I think
is a good idea:

- sub-class Single
- Tweak the result of glient.c.as_jsonish() before the spec string is produced
(gclient/api.py line 245)

I did a quick survey of the use of cache_dir in gclient and git_cache; in all
cases, that code does a normal bool check rather than comparing to None.

I will let you make the call, but IMO the depot_tools code should be able to
handle cache_dir=''; insisting that it be None is fragile and invites future
unimagined-as-yet bugs.

[agable, 2016/03/03 18:11:49]

Actually, in my infinite cynicism, I expect depot_tools to break equally with ''
and None, and to only operate correctly if the key is missing entirely :P

But honestly I think you're right and this is probably totally fine.

     ],
     "cwd": "[SLAVE_BUILD]/src/third_party",
     "env": {