Help minimize export dumps differences when inspecting --dry-run results

When diffing the fast-export dumps produces by `--dry-run`, it appears
that git-filter-repo is using Python's default sorting algorithm for
Git filenames, rather than the fast-export-specific algorithm, which
is implemented at:
https://github.com/git/git/blob/14de3eb34435db79c6e7edc8082c302a26a8330a/builtin/fast-export.c#L444-L448

```
$ ./git-filter-repo --dry-run --proceed
$ diff -u .git/filter-repo/fast-export.original .git/filter-repo/fast-export.filtered
...
@@ -1451,25 +1329,23 @@
 D testcases/expected/case1-twenty
 D testcases/inputs/case1
 M 100755 0a13abf testcases/t9390-repo-filter.sh
+M 100644 de3799f testcases/t9390/case1
 M 100644 e0c8845 testcases/t9390/case1-filename
 M 100644 a1aa78f testcases/t9390/case1-ten
 M 100644 488cbd9 testcases/t9390/case1-twenty
-M 100644 de3799f testcases/t9390/case1
```

Note: this has no consequences on the resulting Git
repository. git-filter-repo doesn't write tree objects directly; it
has git-fast-import do that. git-fast-import states that the order of
the filemodify directives given to it does not matter. This change
only avoids distracting differences when inspecting the modified
fast-export stream.

Signed-off-by: Sylvain Beucler <[email protected]>

Author: Beuc

git-filter-repo

@@ -33,6 +33,7 @@ operations; however:
 import argparse
 import collections
 import fnmatch
+import functools
 import gettext
 import io
 import os
@@ -3940,7 +3941,29 @@ class RepoFilter(object):
         continue
       # Otherwise, record the change
       new_file_changes[change.filename] = change
-    commit.file_changes = [v for k,v in sorted(new_file_changes.items())]
+
+    # Use the git fast-export sorting algorithm for filenames
+    # https://github.com/git/git/blob/14de3eb34435db79c6e7edc8082c302a26a8330a/builtin/fast-export.c#L444-L448
+    def depth_first(a, b):
+      fn_a = a[0]
+      fn_b = b[0]
+
+      # Sort 'd/e' before 'd'
+      # first compare common length, then if equal give priority to longer one
+      min_len = min(len(fn_a), len(fn_b))
+      # memcmp equivalent https://docs.python.org/3.0/whatsnew/3.0.html#ordering-comparisons
+      cmp = (fn_a[:min_len] > fn_b[:min_len]) - (fn_a[:min_len] < fn_b[:min_len])
+      if cmp != 0:  # different content
+        return cmp  # return normal comparison
+      cmp = len(fn_b) - len(fn_a)
+      if cmp != 0:  # different size
+        return cmp  # longer one first
+
+      # 'R' (rename) entries last
+      cmp = (a[1].type == 'R') - (b[1].type == 'R')
+      return cmp
+    commit.file_changes = [v for k,v in sorted(new_file_changes.items(),
+        key=functools.cmp_to_key(depth_first))]
 
   def _tweak_commit(self, commit, aux_info):
     if self._args.replace_message: