convert: fix a file descriptor leak

test-check-code flagged this after I changed this line for something unrelated.

hgext/convert/gnuarch.py

@@ -18,6 +18,7 b' from mercurial import ('
     encoding,
     error,
     pycompat,
+    util,
 )
 from mercurial.utils import (
     dateutil,
@@ -228,7 +229,7 b' class gnuarch_source(common.converter_so'
             else:
                 mode = ''
         else:
-            data = open(os.path.join(self.tmppath, name), 'rb').read()
+            data = util.readfile(os.path.join(self.tmppath, name))
             mode = (mode & 0o111) and 'x' or ''
         return data, mode