Reland of: https://codereview.chromium.org/2049463003/

See https://build.chromium.org/p/tryserver.chromium.win/builders/win10_chromium_x64_rel_ng_exp/builds/1801/steps/bot_update/logs/stdio
[Mirror] https://luci-logdog.appspot.com/v/?s=chromium%2Fbb%2Ftryserver.chromium.win%2Fwin10_chromium_x64_rel_ng_exp%2F1801%2F%2B%2Frecipes%2Fsteps%2Fbot_update%2F0%2Fstdout

Sometimes repos become corrupt and bot_update fails spectacularly.  This usually happens due to the process being interrupted during a fetch. Git
cache should be able to recover from this by clobbering.

BUG=

Review-Url: https://codereview.chromium.org/2045243004