git_cache: add more diagnostic logging wrt bootstrapping.
git_cache: add more diagnostic logging wrt bootstrapping.

This is short of sending this metrics to ts_mon, but is still useful
for diagnose and then claim improvements in git cache performance on
bots.

This change has been tested for realz with led, particularly on windows
https://ci.chromium.org/swarming/task/3a01358af14a7d10

Bug: 749709
Change-Id: I2b3589079d2caa7f70007f90fcbce85a0205d24b
Reviewed-on: https://chromium-review.googlesource.com/787173
Reviewed-by: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@google.com>
Reviewed-by: Sergiy Byelozyorov <aadb4a11584aa9878242ea5d9e4b7e3429654579@chromium.org>
Reviewed-by: Michael Achenbach <76cc7f10de6bc994d83c4cb09072e4590b82b709@chromium.org>
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org>

CoherentLabs/depot_tools,CoherentLabs/depot_tools