filemerge: better handling of failing remove of temporary files

We have seen some failures on Windows that could seem like the unlinks of
temporary files were failing. That could perhaps be because the merge tool
somehow still held the files open.

Instead of the bare bone os.unlink, use our util.unlink with special
rename/retry handling on Windows.

mercurial/filemerge.py

@@ -334,10 +334,10 b' def filemerge(repo, mynode, orig, fcd, f'
             if onfailure:
                 ui.warn(onfailure % fd)
         else:
-            os.unlink(back)
+            util.unlink(back)
 
-        os.unlink(b)
-        os.unlink(c)
+        util.unlink(b)
+        util.unlink(c)
         return r
 
     if not r and (_toolbool(ui, tool, "checkconflicts") or
@@ -368,10 +368,10 b' def filemerge(repo, mynode, orig, fcd, f'
         if onfailure:
             ui.warn(onfailure % fd)
     else:
-        os.unlink(back)
+        util.unlink(back)
 
-    os.unlink(b)
-    os.unlink(c)
+    util.unlink(b)
+    util.unlink(c)
     return r
 
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