RFR: Coalesce hunks that have overlapping context

[Jorn Vernee via github.com]

On Fri, 30 Aug 2019 13:56:52 GMT, Jorn Vernee via github.com <duke at openjdk.java.net> wrote:

> The `webrev` command can generate patch files with adjacent hunks that have overlapping context. `git apply` rejects these patches.
> 
> This PR coalesces adjacent hunks with overlapping context, which is also what's done by `git diff`.
> 
> Also added a test for `HunkCoalescer` (since it's a pretty complex class I think it deserved it's own tests).
> 
> Resulting webrev applies with both git and hg.
> 
> ----------------
> 
> Commits:
>  - 6f5f1071:	Coalesce hunks that have overlapping context, since git rejects these otherwise
> 
> Pull request:
> https://git.openjdk.java.net/skara/pull/113
> 
> Webrev:
> https://webrevs.openjdk.java.net/skara/113/webrev.00
> 
> Patch:
> https://git.openjdk.java.net/skara/pull/113.diff
> 
> Fetch command:
> git fetch https://git.openjdk.java.net/skara pull/113/head:pull/113

@edvbld I tested with the following script:

```powershell
for ($i = 1; $i -le 20; $i++) {
	git webrev -r "HEAD~$i" -N
	git checkout "HEAD~$i"
	git apply .\webrev\git-jdk2.patch
	git checkout master --force
}
```
This went alright until about `HEAD~16`, which threw a 'corrupt patch' error. Looking into it, the offending line is:

    diff a/test/hotspot/jtreg/ProblemList-aot.txt b/test/hotspot/jtreg/ProblemList-aot.txt

I thought it might be the hyphen in the name, but `git diff` generates the same, and that patch does apply cleanly. Even when copying over the line from the git diff to the patch file generated by `webrev` it still complains. Not sure what's going on...

However, running that script without this PR fails on the first webrev, so I'd say that is a net improvement :), and since the issue seems unrelated, I think we could still move ahead with this PR?

PR: https://git.openjdk.java.net/skara/pull/113