Change how the git CLI subprocess is executed

[jcouball]

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TODO

Issue #687 was opened to track that this gem does not work on JRuby for Windows when using JDK > 8. This PR will be merged without this issue being solved.

Waiting for the following issues to be addressed:

• JRuby on Windows
  • Process#spawn should call #to_io on non-IO file descriptor objects jruby/jruby#7565
  • Process.wait and Process.wait2 not behaving as expected on Windows with JDK > 8 jruby/jruby#7515
• Truffle Ruby
  • Process#spawn should call #to_io on non-IO file descriptor objects oracle/truffleruby#2809

Description

Fixes the following issues:

• Using one repo per thread results in errors #355
• archive files generated with archive() may be corrupted #363
• Streaming stdout instead of one response #441
• Commit message escaping issues under Windows #495
• Git.ls_remote fails when stderr isn't empty #516
• Diff parsing fails with opaque error when SSH warnings preceed the standard header #574

In the current implementation, all calls to git are done using backticks which has several disadvantages:

• STDOUT and STDERR are commingled (via redirection added to the command line using 2>&1) which can cause processing errors.
• Changes to the environment's subprocess have to be global which could impact users of this gem
• Arguments passed on the command line go through the system shell making it difficult to escape args correctly for all platforms AND leaving this gem open to security vulnerabilities

Requirements

Implement a cross platform (Windows, Linux, and Mac) and cross Ruby (MRI, JRuby, and TruffleRuby) solution that meets the following requirements:

1. Independently capture STDOUT and STDERR
2. Be able to both capture STDOUT and STDIN as well as stream them to the terminal and a file at the same time if desired (e.g. for debugging)
3. Provide a isolated environment and current working directory for the git command subprocess that does not change these global variables in the process that this gem is running in
4. Pass command line args to the git command without being interpreted by the system shell
5. Be able to implement a timeout on git subprocesses so they do not hang forever if input is asked for

Alternative Implementations Examined

#	Option	Pros	Cons
1	Kernel.` (aka backtick)	Very simple	Does not implement any of the desired reqs
2	IO.popen	Simple	Does not implement desired reqs 1, 2, 5
3	Open3.popen3	Simple	Does not implement desired reqs 2 and 5
4	Open3.capture3	Simple	Does not implement desired reqs 2 and 5
5	Process.spawn	Can be made to implement all reqs	Very complex implementation
6	ProcessExecuter.spawn	Much less complex than Process.spawn. Implements all reqs.	Bespoke wrapper around Process.spawn. Less used in the field than other options.

Option 6 was chosen because it is the least complex implementation that implements all the requirements.

Changes Made in this PR

This PR re-implements Git::Lib#command. In order to get the desired results, some changes were needed to the signature of this method which caused some changes in code and tests.

The implementation of Git::Lib#command is handled by the new class Git::CommandLine. This class has a small interface: #initialize which accepts attribute values that do not change between git command invocations (env, binary_path, global_opts, and logger) and #run which takes the git command args, redirection, and flags for normalization, chomping, and merging stderr with stdout.