This partially reverts commit d51ed57e.

Reason for revert:
New git client for windows was rolled including fix for slow `git fetch`.
I guess smaller pack limit causes frequent bootstrap taking 2~3 minutes longer than
the case it does not happen.
Let me see what happen if we increase pack limit 9 -> 30.
I will increase this to 50 if this won't cause regression again.

Original change's description:
> git_cache: lower max num of .pack files before re-bootstrap on Win.
>
> It used to be 50, I think ~9 gives best results for Chromium on Win:
>   on golo VM, it takes <4 minutes to re-boostrap + git fetch small
>   delta, assuming zipped git checkout for bootstrap is fresh (~1day).
>
> For other repos, which are significantly smaller, this change should
> have minor effect if at all.
>
> Test: I tested this using `led` tool on Win7 machines running LUCI
> stack extensively. For example,
>
>   * https://ci.chromium.org/swarming/task/3a0102e8c8657410
>     shows case with few .pack files, hence just 1 fetch
>
>   * https://ci.chromium.org/swarming/task/3a010282f9fd8010
>     shows case with 39 .pack files and so bootstrapping + fetch.
>     If you look at prior tasks on the same VM, you'd find this:
>     https://ci.chromium.org/swarming/task/39ffe843d01ed010
>     which spent 8 minutes doing 1 incremental fetch with 39 .pack
>     files.
>
> **Troopers/Sheriffs**: This change is safe to revert.
> However, beware that you should also at the same time revert the recipe
> roll of this CL to the repo, in which the failed builder's recipe is
> located, typically `chromium/tools/build`.
>
> Bug: 749709
> Change-Id: I18e2b63283100d466e9fb981a9094862463f6909
> Reviewed-on: https://chromium-review.googlesource.com/787174
> Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>
> Reviewed-by: Takuto Ikuta <tikuta@google.com>

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 749709
Change-Id: I3052abe4a9b53277a60c0791a85355e7a0bbdf8f
Reviewed-on: https://chromium-review.googlesource.com/823544
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Takuto Ikuta <tikuta@google.com>