Use subprocess.Popen to call git executables

Use subprocess.Popen to call git executables

Replace popen2.Popen3 and os.spawn* with the superior
subprocess.Popen. Now we can pass a modified environment to any
subprocess, redirect input and output as we please, and the shell is
no longer involved, meaning we don't have to worry about argument
quoting. (We could already do all of that, just not at the same time.)

This is a Python 2.4 library, so StGit now officially requires Python
2.4 or later.

Signed-off-by: Karl Hasselström <kha@treskal.com>