Improved unified diff compatibility for --diff option

[rg3igalia]

Universal diffs as generated by git (for example) have some compatibility problems regarding what lcov --diff expects. This pull request fixes a few of those problems to make it easier to use git diffs. In particular:

• Start-of-block lines are now allowed to have comments after the ending @@. Git sometimes uses this to give hints about the diff block location, like the function name.
• The diff reader has been changed to track more state and to be a bit more flexible, due to git printing more info lines in between files and in the header, like when using git show.
• In addition, I've added an .editorconfig file that can be used to maintain some style when using different editors like Vim, Emacs, VS Code, etc.
• Thanks to the previous point, I've also fixed some minor whitespace issues.

[henry2cox]

I was going though old PRs and found this one. 4 years is a long time - sorry.

In the meantime, the lcov code has been refactored quite a bit.
Merging this PR is pretty straightforward, as the only changes we want are in the 'read_diff' function.
There - apart from reformatting via make checkstyle MODE=full UPDATE=1 (to match lcov coding standard) - the only change that is needed is to call 'lcovutil::strip_directories' rather than 'strip_directories')

The other thing that is needed is a 'Signed-off-by:' line in your commit/PR.
I can do both of those things - if I knew your email address for the signoff line.

[rg3igalia]

Oh, thanks for reviewing! Better late than ever. If you could take care of both things, it would be great. My email address is rgarcia AT igalia DOT com.

[henry2cox]

above commit pushed the refactored PR.

Thank you for your contribution.
Henry

[rg3igalia]

You're welcome and thanks for merging!