Cleaned up various readPipe() threading issues

Cleaned up various readPipe() threading issues

Fixed random errors in discoverGitSystemConfig() on Linux where the
process error stream was closed by readPipe() before or while
GobblerThread was reading from it.

Marked readPipe() as @Nullable and fixed potential NPE in
discoverGitSystemConfig() on readPipe() return value.

Fixed process error output randomly mixed with other threads log
messages.

Change-Id: Id882af2762cfb75f010f693b2e1c46eb6968ee82
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>