base85: avoid a spurious use-before-initialized warning in `pure` module

The error wasn't possible because the only way for `acc` to not be initialized
was if `len(text) == 0`.  But then `0 % 5 == 0`, so no attempt at padding was
done.  It's a simple enough fix to not have PyCharm flag this though.  The value
needs to be reset on each loop iteration, so it's a line copy, not a line move.

mercurial/pure/base85.py

@@ -58,6 +58,8 def b85decode(text: bytes) -> bytes:
 
     l = len(text)
     out = []
+    acc = 0
+
     for i in range(0, len(text), 5):
         chunk = text[i : i + 5]
         chunk = pycompat.bytestr(chunk)