revlog: avoid string slice when decompressing u* chunks

Revlog chunks can be stored uncompressed. If the first byte of the
raw data is \0, we store the data as is. Else we prefix it with 'u'.

Before, we performed a string slice to strip out the 'u' prefix.
With this patch, we use a buffer to avoid an extra memory copy and
associated garbage collection overhead.

I was unable to verify any performance impact of this patch. For both
mozilla-central and the hg repos, the number of manifest revisions
with 'u' prefixes is very small - under 1%. So this change likely
isn't called enough to have an impact on manifest reading. However,
the reasoning behind this change is solid, so it should be safe.

mercurial/revlog.py

@@ -110,7 +110,7 b' def decompress(bin):'
         except zlib.error as e:
             raise RevlogError(_("revlog decompress error: %s") % str(e))
     if t == 'u':
-        return bin[1:]
+        return util.buffer(bin, 1)
     raise RevlogError(_("unknown compression type %r") % t)
 
 # index v0: