Minor improvements in git config file inclusions

Minor improvements in git config file inclusions

* Section and key names in git config files are case-insensitive.
* If an include directive is invalid, include the line in the
  exception message.
* If inclusion of the included file fails, put the file name into
  the exception message so that the user knows in which file the
  problem is.

Change-Id: If920943af7ff93f5321b3d315dfec5222091256c
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>