add: introduce add.ignoreerrors synonym for add.ignore-errors

add: introduce add.ignoreerrors synonym for add.ignore-errors

The "[add] ignore-errors" tweakable introduced by v1.5.6-rc0~30^2 (Add
a config option to ignore errors for git-add, 2008-05-12) does not
follow the usual convention for naming values in the git configuration
file.

What convention?  Glad you asked.

	The section name indicates the affected subsystem.

	The subsection name, if any, indicates which of
	an unbound set of things to set the value for.

	The variable name describes the effect of tweaking
	this knob.

	The section and variable names can be broken into
	words using bumpyCaps in documentation as a hint to
	the reader.  These word breaks are not significant
	at the level of code, since the section and variable
	names are not case sensitive.

The name "add.ignore-errors" includes a dash, meaning a naive
configuration file like

	[add]
		ignoreErrors

does not have any effect.  Avoid such confusion by renaming to the
more consistent add.ignoreErrors, but keep the old version for
backwards compatibility.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>