This reverts commit 6e215cef.

Reason for revert: seems to have broken bot_update 
https://ci.chromium.org/swarming/task/39ff1814b3bb5b10?server=chromium-swarm.appspot.com

Original change's description:
> Show GIT_TRACE for git command in bot_update.py
> 
> GIT_TRACE=true tells us which internal git command is slow.
> https://git-scm.com/book/en/v2/Git-Internals-Environment-Variables
> 
> I got following results in my local repository.
> Let me confirm the bots show similar behavior.
> 
> C:\src\chromium\src>set GIT_TRACE=true
> 
> C:\src\chromium\src>git rebase-update
> 21:32:36.519698 git.c:576               trace: exec: 'git-rebase-update'
> 21:32:36.519698 run-command.c:626       trace: run_command: 'git-rebase-update'
> 21:32:38.889848 git.c:344               trace: built-in: git 'fetch' '--multiple' 'origin'
> 21:32:38.893846 run-command.c:626       trace: run_command: 'fetch' '--append' 'origin'
> Fetching origin
> 21:32:38.911858 git.c:344               trace: built-in: git 'fetch' '--append' 'origin'
> 21:32:38.983909 run-command.c:626       trace: run_command: 'remote-https' 'origin' 'https://chromium.googlesource.com/chromium/src.git'
> 21:32:39.002923 git.c:576               trace: exec: 'git-remote-https' 'origin' 'https://chromium.googlesource.com/chromium/src.git'
> 21:32:39.003923 run-command.c:626       trace: run_command: 'git-remote-https' 'origin' 'https://chromium.googlesource.com/chromium/src.git'
> 21:32:43.782266 run-command.c:626       trace: run_command: 'rev-list' '--objects' '--stdin' '--not' '--all' '--quiet'
> 21:32:43.827301 run-command.c:626       trace: run_command: 'fetch-pack' '--stateless-rpc' '--stdin' '--lock-pack' '--include-tag' '--thin' 'https://chromium.googlesource.com/chromium/src.git/'
> remote: Counting objects: 74400, done
> remote: Finding sources: 100% (19/19)
> 21:35:49.348951 run-command.c:626       trace: run_command: 'unpack-objects' '--pack_header=2,19'
> 21:35:49.368985 git.c:344               trace: built-in: git 'unpack-objects' '--pack_header=2,19'
> remote: Total 19 (delta 1), reused 18 (delta 1)
> Unpacking objects: 100% (19/19), done.
> 21:35:49.597328 run-command.c:626       trace: run_command: 'rev-list' '--objects' '--stdin' '--not' '--all' '--quiet'
> 21:35:49.612375 git.c:344               trace: built-in: git 'rev-list' '--objects' '--stdin' '--not' '--all' '--quiet'
> From https://chromium.googlesource.com/chromium/src
>    e8994685157b..fd56d3ad0528  master     -> origin/master
> 21:35:50.281845 run-command.c:1452      run_processes_parallel: preparing to run up to 1 tasks
> 21:35:50.287848 run-command.c:1484      run_processes_parallel: done
> 21:35:50.309865 run-command.c:626       trace: run_command: 'gc' '--auto'
> 21:35:50.328880 git.c:344               trace: built-in: git 'gc' '--auto'
> 21:35:50.453967 run-command.c:1452      run_processes_parallel: preparing to run up to 1 tasks
> 21:35:50.460971 run-command.c:1484      run_processes_parallel: done
> 21:35:50.460971 run-command.c:626       trace: run_command: 'gc' '--auto'
> 21:35:50.480985 git.c:344               trace: built-in: git 'gc' '--auto'
> Rebasing: direct_goma
> Rebasing: python_pool
> 
> C:\src\chromium\src>git fetch
> 21:36:23.409703 git.c:344               trace: built-in: git 'fetch'
> 21:36:23.486758 run-command.c:626       trace: run_command: 'remote-https' 'origin' 'https://chromium.googlesource.com/chromium/src.git'
> 21:36:23.506772 git.c:576               trace: exec: 'git-remote-https' 'origin' 'https://chromium.googlesource.com/chromium/src.git'
> 21:36:23.506772 run-command.c:626       trace: run_command: 'git-remote-https' 'origin' 'https://chromium.googlesource.com/chromium/src.git'
> 21:36:28.542596 run-command.c:626       trace: run_command: 'rev-list' '--objects' '--stdin' '--not' '--all' '--quiet'
> 21:36:28.578620 run-command.c:626       trace: run_command: 'fetch-pack' '--stateless-rpc' '--stdin' '--lock-pack' '--include-tag' '--thin' 'https://chromium.googlesource.com/chromium/src.git/'
> 21:36:28.601635 git.c:344               trace: built-in: git 'fetch-pack' '--stateless-rpc' '--stdin' '--lock-pack' '--include-tag' '--thin' 'https://chromium.googlesource.com/chromium/src.git/'
> remote: Counting objects: 67598, done
> remote: Finding sources: 100% (28/28)
> 21:39:29.787441 run-command.c:626       trace: run_command: 'unpack-objects' '--pack_header=2,28'
> 21:39:29.812462 git.c:344               trace: built-in: git 'unpack-objects' '--pack_header=2,28'
> remote: Total 28 (delta 2), reused 20 (delta 2)
> Unpacking objects: 100% (28/28), done.
> 21:39:30.083690 run-command.c:626       trace: run_command: 'rev-list' '--objects' '--stdin' '--not' '--all' '--quiet'
> 21:39:30.108704 git.c:344               trace: built-in: git 'rev-list' '--objects' '--stdin' '--not' '--all' '--quiet'
> From https://chromium.googlesource.com/chromium/src
>    fd56d3ad0528..fbcc09282dc8  master     -> origin/master
> 21:39:30.788186 run-command.c:1452      run_processes_parallel: preparing to run up to 1 tasks
> 21:39:30.795191 run-command.c:1484      run_processes_parallel: done
> 21:39:30.813200 run-command.c:626       trace: run_command: 'gc' '--auto'
> 21:39:30.836212 git.c:344               trace: built-in: git 'gc' '--auto'
> 
> 
> Bug:749709
> Change-Id: I5b94549c42271885365c2354dc3f58399d6aebe0
> Reviewed-on: https://chromium-review.googlesource.com/786691
> Reviewed-by: Ryan Tseng <hinoka@chromium.org>
> Commit-Queue: Takuto Ikuta <tikuta@google.com>

TBR=hinoka@chromium.org,agable@chromium.org,tandrii@chromium.org,tikuta@google.com

Change-Id: I970ddac1fd21e904c69dc18e48169d49293b1800
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 749709
Reviewed-on: https://chromium-review.googlesource.com/786413Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>