Our rbtools.hooks.common.execute() function uses subprocess.Popen and

Popen.communicate to execute commands and read their output. Between Python 2

and 3, subprocess.Popen(...).communicate() changed from returning a tuple of

strings to a tuple of bytes. This breaks some of our hook util methods in

rbtools.hooks.git where we try to perform string operations on the output

of execute(), but we receive bytes instead of strings.
Python 3.7 added a text parameter to Popen to control whether file

objects, stdin, stdout, and stderr are opened in text mode or binary mode.

We make use of that parameter by adding our own text parameter to execute()

to control whether to return the stdout output as a string or bytes. This

defaults to False to match the default behavior of Popen in Python 3.
This also makes it optional to strip whitespace from the output, instead of

always stripping it. This is useful when dealing with commands where we

need to preserve the whitespace in the output, such as with git diff.
This changes also deprecates the rbtools.hooks.common.execute() function

in favour of rbtools.utils.process.run_process(), which is more

future-proof and has better type safety.

Ran into an error when running the git pre-receive hook, which came

  from the clash between string and bytes. Applied this fix and ran

  the hook again, saw that the error was gone.
Ran unit tests.