add a test for semantic errors in config files

add a test for semantic errors in config files

Semantic errors (for example, for alias.* variables NULL values are
not allowed) in configuration files cause a die printing the line
number and file name of the offending value.

Add a test documenting that such errors cause a die printing the
accurate line number and file name.

Signed-off-by: Tanay Abhra <redacted>
Reviewed-by: Matthieu Moy <redacted>
Signed-off-by: Junio C Hamano <redacted>