Config load should not fail on unsupported or nonexistent include path

Config load should not fail on unsupported or nonexistent include path

1f86350 added initial support for include.path. Relative path and path
with tilde are not yet supported but config load was failing if one of
those 2 unsupported options was encountered. Another problem was that
config load was failing if the include.path file did not exist.

Change the behavior to be consistent with native git. Ignore unsupported
or nonexistent include.path.

Bug: 495505
Bug: 496732
Change-Id: I7285d0e7abb6389ba6983e9c46021bea4344af68
Signed-off-by: Hugo Arès <hugo.ares@ericsson.com>