Improve detection of input/output file name matching

[fulldecent]

Reading from and writing to the same file should be supported

[lovell]

Hi, sorry, this is due to a performance benefit of libvips, which allows it to start writing to the output file whilst still reading the input file.

[fulldecent]

That's cool.

Then could it please fail gracefully rather than clobbering the file?

[lovell]

There is some simple logic to check for this:

sharp/lib/output.js

Lines 57 to 58 in e044788

	} else if (this.options.input.file === fileOut) {
	err = new Error('Cannot use same file for input and output');

Perhaps this should use something like path.resolve to handle relative paths? Happy to accept a PR for this, if you're able.

[fulldecent]

This is great, thank you!

[lovell]

v0.29.1 now available.