Revert of Refactor source_control.py and add a test.

Revert of Refactor source_control.py and add a test. (patchset #3 id:40001 of https://codereview.chromium.org/650223005/)

Reason for revert:
This broke syncing source for dependency repos while bisecting them, 
http://build.chromium.org/p/tryserver.chromium.perf/builders/android_nexus7_perf_bisect/builds/384/steps/Working%20on%20%5Bc519ca88f8ed073ff523f03eee8cb05d8b848fc5%5D/logs/stdio

Original issue's description:
> Refactor source_control.py and add a test.
> 
> Originally, Simon made SourceControl a class becaused we expected to
> add a SvnSourceControl subclass which implemented the same methods in
> different ways. However, after the git transition we're fairly sure
> that we don't want to do this.
> 
> So, all of the functions in the SourceControl class can be made top-level
> functions that don't need a "self" argument. This way, there's no need
> to pass around a bogus SourceControl object; the functions can be called
> directly.
> 
> I also added tests for some methods that seemed relatively testable.
> 
> PTAL and give suggestions :-)
> 
> BUG=
> 
> Committed: https://crrev.com/880741668d3ed0e08ea2229bb8965998abe775a3
> Cr-Commit-Position: refs/heads/master@{#299851}

TBR=ojan@chromium.org,qyearsley@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=

[prasadv, 2014-10-20 17:11:33]

Created Revert of Refactor source_control.py and add a test.

[qyearsley, 2014-10-20 17:54:18]

On 2014/10/20 17:11:33, prasadv wrote:
> Created Revert of Refactor source_control.py and add a test.

Note: This revert may not be necessary (and therefore could be closed) if the
patch CL https://codereview.chromium.org/662403003/ works.