extdiff: move single file handling inside `not per-file` conditional

If `--per-file` is passed, we will diffing files one by one, hence there is no
need to make this optimization around single file in that case. This
optimization is only required when `--per-file` is not passed and we will be
opening directory-diff for a single file.

Differential Revision: https://phab.mercurial-scm.org/D8971

hgext/extdiff.py

@@ -456,6 +456,7 b' def diffrevs('
     label1b = rev1b
     label2 = rev2
 
+    if not opts.get(b'per_file'):
     # If only one change, diff the files instead of the directories
     # Handle bogus modifies correctly by checking if the files exist
     if len(common) == 1:
@@ -472,7 +473,6 b' def diffrevs('
         dir2 = os.path.join(dir2root, dir2, common_file)
         label2 = common_file + rev2
 
-    if not opts.get(b'per_file'):
         # Run the external tool on the 2 temp directories or the patches
         cmdline = formatcmdline(
             cmdline,