Fix UnboundLocalError with plugins and largefiles

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)

README.md

@@ -188,7 +188,7 @@ values in the dictionary after filters have been run are used to create the git
 commit.
 
 ```
-file_data = {'filename':filename,'file_ctx':file_ctx,'data':file_contents}
+file_data = {'filename':filename,'file_ctx':file_ctx,'data':file_contents, 'is_largefile':largefile_status}
 
 def file_data_filter(self,file_data):
 ```
@@ -203,6 +203,12 @@ but 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 when files are deleted.
 
+The `is_largefile` entry within the `file_data` dictionary will contain
+`True` if the original file was a largefile and has been converted
+to a normal file before the plugins were invoked. In this case, the `file_ctx`
+will still point to the filecontext for the original, unconverted file, while
+`filename` and `data` will contain the already converted information.
+
 Submodules
 ----------
 See README-SUBMODULES.md for how to convert subrepositories into git

hg-fast-export.py

@@ -211,15 +211,18 @@ def export_file_contents(ctx,manifest,files,hgtags,encoding='',plugins={}):
         b'Ignoring file %s which cannot be tracked by git\n' % filename
       )
       continue
+
+    largefile = False
+    file_ctx=ctx.filectx(file)
     if is_largefile(filename):
+      largefile = True
       filename = largefile_orig_name(filename)
       d = largefile_data(ctx, file, filename)
     else:
-      file_ctx=ctx.filectx(file)
       d=file_ctx.data()
 
     if plugins and plugins['file_data_filters']:
-      file_data = {'filename':filename,'file_ctx':file_ctx,'data':d}
+      file_data = {'filename':filename,'file_ctx':file_ctx,'data':d, 'is_largefile':largefile}
       for filter in plugins['file_data_filters']:
         filter(file_data)
       d=file_data['data']

t/largefile_plugin.expected

@@ -0,0 +1,20 @@
+blob
+mark :1
+data 7
+a_file
+
+blob
+mark :2
+data 6
+large
+
+reset refs/heads/master
+commit refs/heads/master
+mark :3
+author Grevious Bodily Harmsworth <gbh@example.com> 1679014800 +0000
+committer Grevious Bodily Harmsworth <gbh@example.com> 1679014800 +0000
+data 3
+r0
+M 100644 :1 a.txt
+M 100644 :2 b.txt
+

t/largefile_plugin.t

@@ -0,0 +1,69 @@
+#!/bin/bash
+#
+# Copyright (c) 2023 Felipe Contreras
+# Copyright (c) 2023 Frej Drejhammar
+# Copyright (c) 2025 Günther Nußmüller
+#
+# Check that plugin invocation works with largefiles.
+# This test uses the echo_file_data_test_plugin to verify that the
+# file data is passed correctly, including the largefile status.
+#
+
+test_description='Largefiles and plugin test'
+
+. "${SHARNESS_TEST_SRCDIR-$(dirname "$0")/sharness}"/sharness.sh || exit 1
+
+
+git_create() {
+	git init -q "$1" &&
+	git -C "$1" config core.ignoreCase false
+}
+
+git_convert() {
+	(
+	cd "$2" &&
+	hg-fast-export.sh --repo "../$1" \
+			  -s --hgtags -n \
+			  --plugin ../../plugins/echo_file_data_test_plugin
+	)
+}
+
+setup() {
+	cat > "$HOME"/.hgrc <<-EOF
+	[ui]
+	username = Grevious Bodily Harmsworth <gbh@example.com>
+	[extensions]
+	largefiles =
+	EOF
+}
+
+commit0() {
+	(
+	cd hgrepo &&
+	echo "a_file" > a.txt &&
+	echo "large" > b.txt
+	hg add a.txt &&
+	hg add --large b.txt &&
+	hg commit -d "2023-03-17 01:00Z" -m "r0"
+	)
+}
+
+setup
+
+test_expect_success 'largefile and plugin' '
+	test_when_finished "rm -rf hgrepo gitrepo" &&
+
+	(
+	hg init hgrepo &&
+	commit0
+	) &&
+	git_create gitrepo &&
+	git_convert hgrepo gitrepo &&
+	
+	git -C gitrepo fast-export --all > actual &&
+
+ 	test_cmp "$SHARNESS_TEST_DIRECTORY"/largefile_plugin.expected actual &&
+	test_cmp "$SHARNESS_TEST_DIRECTORY"/largefile_plugin_file_info.expected gitrepo/largefile_info.txt
+'
+
+test_done

t/largefile_plugin_file_info.expected

@@ -0,0 +1,12 @@
+filename: b'b.txt'
+data size: 6 bytes
+ctx rev: 0
+ctx binary: False
+is largefile: True
+
+filename: b'a.txt'
+data size: 7 bytes
+ctx rev: 0
+ctx binary: False
+is largefile: False
+

t/plugins/echo_file_data_test_plugin/__init__.py

@@ -0,0 +1,18 @@
+import sys
+from mercurial import node
+
+def build_filter(args):
+    return Filter(args)
+
+class Filter:
+    def __init__(self, _):
+        pass
+
+    def file_data_filter(self,file_data):
+        with open('largefile_info.txt', 'a') as f:
+            f.write(f"filename: {file_data['filename']}\n")
+            f.write(f"data size: {len(file_data['data'])} bytes\n")
+            f.write(f"ctx rev: {file_data['file_ctx'].rev()}\n")
+            f.write(f"ctx binary: {file_data['file_ctx'].isbinary()}\n")
+            f.write(f"is largefile: {file_data.get('is_largefile', False)}\n")
+            f.write("\n")