add_codereview_message script to append messages to a CL

Add add_codereview_message.py script.  This script takes a message and
a codereview URL as arguments calls `git cl upload` to append the
message to the given codereview issue.

Motivation: We are automating a process of testing Chromium/Skia DEPS
rolls where roll_deps.py is automatically run every night, and then at
a later time, compare_codereview.py is run to generate a report of the
trybot results.  This script can be used to append that report to the
DEPS roll Cl.

This CL also refactors functioanlity common to roll_deps and
add_codereview_message:
*   Add tools/git_utils.py module.
    -   ChangeGitBranch class was factored out of
        roll_deps.GitBranchCLUpload.  The other half of that class
        became roll_deps.git_cl_uploader function.  I make use of
        this new class in both roll_deps and upload_deps_roll.
    -   test_git_executable moved to this new module.
*   Add tools/misc_utils.py - move VerboseSubprocess, ChangeDir,
    and ReSearch classes from roll_deps module.
*   Remove generic functions from roll_deps.
*   Add git_cl_uploader function to roll_deps.  Refactor
    roll_deps.roll_deps function to make use of it

BUG=skia:
R=borenet@google.com, robertphillips@google.com

Author: halcanary@google.com

Review URL: https://codereview.chromium.org/141483011

git-svn-id: http://skia.googlecode.com/svn/trunk@13251 2bbb7eff-a529-9590-31e7-b0007b416f81

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