bot_update: fix git dir check

bot_update: fix git dir check

existance of a dir does not mean it is a git repo. Check .git dir
presence instead.

This is the reason why swarmbucket build.git builds fail.

TBR=hinoka@chromium.org
BUG=

Committed: https://chromium.googlesource.com/chromium/tools/depot_tools/+/cdac80c9fb4768bdc5bc42734e852e1f216224c9

[nodir, 2016-07-22 19:03:08]

hinoka lgtmed it offline, so landing

[nodir, 2016-07-22 19:03:12]

The CQ bit was checked by nodir@chromium.org

[commit-bot: I haz the power, 2016-07-22 19:03:21]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-07-22 19:05:57]

Description was changed from

==========
bot_update: fix git dir check

existance of a dir does not mean it is a git repo. Check .git dir
presence instead.

This is the reason why swarmbucket build.git builds fail.

TBR=hinoka@chromium.org
BUG=
==========

to

==========
bot_update: fix git dir check

existance of a dir does not mean it is a git repo. Check .git dir
presence instead.

This is the reason why swarmbucket build.git builds fail.

TBR=hinoka@chromium.org
BUG=

Committed:
https://chromium.googlesource.com/chromium/tools/depot_tools/+/cdac80c9fb4768...
==========

[commit-bot: I haz the power, 2016-07-22 19:05:58]

Committed patchset #1 (id:1) as
https://chromium.googlesource.com/chromium/tools/depot_tools/+/cdac80c9fb4768...

[nodir, 2016-07-22 22:48:59]

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/2173683003/ by nodir@chromium.org.

The reason for reverting is: this is wrong. if directory is not empty, git clone
fails.