tests/test-batching.py.out

perf: introduce safeattrsetter to replace direct attribute assignment

Referring not-existing attribute immediately causes failure, but
assigning a value to such attribute doesn't.

For example, perf.py has code paths below, which assign a value to
not-existing attribute. This causes incorrect performance measurement,
but these code paths are executed successfully.

  - "repo._tags = None" in perftags()
    recent Mercurial has tags cache information in repo._tagscache

  - "branchmap.write = lambda repo: None" in perfbranchmap()
    branchmap cache is written out by branchcache.write() in branchmap.py

"util.safehasattr() before assignment" can avoid this issue, but might
increase mistake at "copy & paste" attribute name or so.

To centralize (1) examining existence of, (2) assigning a value to,
and (3) restoring an old value to the attribute, this patch introduces
safeattrsetter(). This is used to replace direct attribute assignment
in subsequent patches.

Encapsulation of restoring is needed to completely remove direct
attribute assignment from perf.py, even though restoring isn't needed
so often.

== Local
Ready.
Un and Deux
Eins und Zwei
One and Two
Nope
Eins und Zwei
Hello, John Smith
Ready.
Uno und Due

== Remote
Ready.
REQ: foo?one=Vo&two=Efvy
  -> Vo!boe!Efvy
Un and Deux
REQ: bar?b=Fjot&a=[xfj
  -> Fjot!voe![xfj
Eins und Zwei
REQ: batch?cmds=foo:one=Pof,two=Uxp;bar:b=Fjot,a=[xfj
  -> Pof!boe!Uxp;Fjot!voe![xfj
REQ: greet?name=Kpio!Tnjui
  -> Ifmmp-!Kpio!Tnjui
REQ: batch?cmds=bar:b=Vop,a=Evf
  -> Vop!voe!Evf
One and Two
Nope
Eins und Zwei
Hello, John Smith
Ready.
Uno und Due