py3: fix crash when server address is 0.0.0.0 (issue6362)

`socket.getfqdn()` assumes that the name is passed as `str` on Python 3 and
always returns `str` in this case. Mercurial passed `bytes` (but still expected
a `str` result), which worked by chance in many cases, except for e.g.
b'0.0.0.0', which was returned unchanged, breaking later code.

Instead of calling `socket.getfqdn()`, we can also use `self.server_name` from
the base `HTTPServer` class, which already stores the FQDN of the locally-bound
socket name (see `BaseHTTPServer.py` in the Python 2 stdlib and
`http/server.py` in the Python 3 stdlib).

mercurial/hgweb/server.py

@@ -382,7 +382,7 b' class MercurialHTTPServer(_mixin, httpse'
         self.errorlog = elog
 
         self.addr, self.port = self.socket.getsockname()[0:2]
-        self.fqaddr = socket.getfqdn(addr[0])
+        self.fqaddr = self.server_name
 
         self.serverheader = ui.config(b'web', b'server-header')
 

tests/test-serve.t

@@ -99,4 +99,11 b' With out of bounds accesses'
   $LOCALIP - - [$LOGDATE$] "GET /some/dir7?cmd=capabilities HTTP/1.1" 404 - (glob)
   $LOCALIP - - [$LOGDATE$] "GET /some?cmd=capabilities HTTP/1.1" 404 - (glob)
 
+  $ "$PYTHON" $RUNTESTDIR/killdaemons.py $DAEMON_PIDS
+
+issue6362: Previously, this crashed on Python 3
+
+  $ hg serve -a 0.0.0.0 -d
+  listening at http://*:$HGPORT1/ (bound to *:$HGPORT1) (glob)
+
   $ cd ..