Add options which allows us to use difft as Apache Subversion's external diff tool

[futatuki]

svn, Apache Subversion's command line client also has option to use external diff tool for svn diff subcommand. However it assumes that diff tool has diff compatible -L (label) and -u option.

This implements -L and -u options.

[Wilfred]

Neat, it'd be nice to have svn support.

I'm OK with adding label options, but I don't like having multiple ways of setting the display mode.

Could we support this syntax similar to git support? If it's always the same arguments in the same order, we could allow those arguments without allowing -u in all circumstances.

[futatuki]

Thank you for being interested in this PR.

-u option is passed only if the user does not specify any additional options. This is done by svn_io_run_diff2() C API function. In the case, -u options is always the first argument, 0 - 2 -L LABEL options is followed and then finally 2 paths, however this order is not documented.

How about hiding -u option from help and just ignore it in the argment processing ?

[bmarwell]

I'd be interested!

[bmarwell]

@futatuki can you rebase your diff, please?

[futatuki]

Yes, I'm willing to do later, with pleasure.

[bmarwell]

Yes, I'm willing to do later, with pleasure.

Thanks! I am using your patch, it is really helpful. I hope @Wilfred sees this in time and can maybe integrate it :)

[futatuki]

Rebase done.