Cannot read virtual files while 'git mergetool` is running

The CanCreatePlaceholder check:

https://github.com/microsoft/VFSForGit/blob/22582651bfd96c936ef609ec2fa76c4f75ddb66d/GVFS/GVFS.Platform.Windows/WindowsFileSystemVirtualizer.cs#L385

In GetPlaceholderInfoCallback is preventing users from being able to read virtual files while git mergetool is running.

To reproduce issue:

1. Push two branches that will have a merge conflict. One of the two branches should contain source files not present in the other.
2. Create a new clone of the repo
3. Checkout the first branch (that does not have the new files)
4. Merge in the second branch
5. After the merge conflict occurs, run git mergetool
6. While the merge tool is running, attempt to read one of the new files (or any file that does not currently have a placeholder on disk). The read will fail.

Example verbose logging during step 6:

[2019-05-09 15:16:25 -07:00] GetPlaceholderInfoCallback {"Area":"WindowsFileSystemVirtualizer","relativePath":"1.txt","commandId":364,"triggeringProcessId":24464,"triggeringProcessImageFileName":"\Device\HarddiskVolume9\Windows\System32\notepad.exe","Message":"GetPlaceholderInfoCallback: Not allowed to create placeholder"}

See also related issues #1040, #1096

Start at GVFS/GVFS.Platform.Windows/WindowsFileSystemVirtualizer.cs near the linked CanCreatePlaceholder check and trace how GetPlaceholderInfoCallback handles reads during git mergetool. Reproduce the documented conflicting-branch sequence, then verify that a file without an on-disk placeholder can be read while the merge tool is running.