Bug 283337 – export-w3c-test-changes should have a better error message for merge conflicts

• Status: NEW
• Component: Tools / Tests

[Karl Dubost]

Let's say you have an older version of the tests on the WebKit repo than the ones on web-platform-tests.
The user merged the code on WebKit side, but when it's time to export the changes, this doesn't work because there is a merge conflict. 


```
karlcow@Oison OpenSource % export-w3c-test-changes -c -g HEAD -b 280749 -d /Volumes/cidrerie/code/web-platform-tests
Fetching web-platform-tests repository
Cleaning web-platform-tests master branch
Applying patch to web-platform-tests branch wpt-export-for-webkit-280749
WARNING: Failed to run "['git', 'apply', '--index', '/var/folders/fs/72c82yrd15sfs1sqp_yf8zh80000gn/T/tmpnjqk_f6fwpt_export_patch', '-3']" exit_code: 1 cwd: /Volumes/cidrerie/code/web-platform-tests
ERROR: Cannot create web-platform-tests local branch from the patch '/var/folders/fs/72c82yrd15sfs1sqp_yf8zh80000gn/T/tmpnjqk_f6fwpt_export_patch'
Keeping local branch wpt-export-for-webkit-280749
```


looking at the web-platform-test repo

```
karlcow@Oison web-platform-tests % git status
On branch wpt-export-for-webkit-280749
Changes to be committed:
  (use "git restore --staged <file>..." to unstage)
	modified:   clipboard-apis/async-navigator-clipboard-read-resource-load.https.html

Unmerged paths:
  (use "git restore --staged <file>..." to unstage)
  (use "git add <file>..." to mark resolution)
	both modified:   clipboard-apis/async-html-script-removal.https.html
```


Is probably a sign of a merge conflict for the export. 

```
WARNING: Failed to run "['git', 'apply', '--index', '/var/folders/fs/72c82yrd15sfs1sqp_yf8zh80000gn/T/tmpnjqk_f6fwpt_export_patch', '-3']" exit_code: 1 cwd: /Volumes/cidrerie/code/web-platform-tests
```

[Radar WebKit Bug Importer]

<rdar://problem/140559776>