When Plugins are used in a repository that contains largefiles,
the following exception is thrown as soon as the first largefile
is converted:
```
Traceback (most recent call last):
File "fast-export/hg-fast-export.py", line 728, in <module>
sys.exit(hg2git(options.repourl,m,options.marksfile,options.mappingfile,
File "fast-export/hg-fast-export.py", line 581, in hg2git
c=export_commit(ui,repo,rev,old_marks,max,c,authors,branchesmap,
File "fast-export/hg-fast-export.py", line 366, in export_commit
export_file_contents(ctx,man,modified,hgtags,fn_encoding,plugins)
File "fast-export/hg-fast-export.py", line 222, in export_file_contents
file_data = {'filename':filename,'file_ctx':file_ctx,'data':d}
UnboundLocalError: local variable 'file_ctx' referenced before assignment
```
This commit fixes the error by:
* initializing the file_ctx before the largefile handling takes place
* Providing a new `is_largefile` value for plugins so they can detect
if largefile handling was applied (and therefore the file_ctx
object may no longer be in sync with the git version of the file)
The `file_data_filter` method should be called when files are deleted.
In this case the `data` and `file_ctx` keys map to None. This is so
that a filter which modifies file names can apply the same name
transformations before files are deleted.
