overrides/Gio: Guard against repeated _promisify() calls

Calling _promisify() on a method that is already promisified will
break it. Fix this with a simple check at the start of the function,
and bail out if the method has been promisifed before.
18 jobs for fix-re-promisify in 11 minutes and 23 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Source Check
passed #641182
build_recommended

00:05:53

passed #645356
cppcheck

00:00:29

passed #641184
cpplint

00:01:34

passed #641185
eslint

00:00:51

passed #645355
cppcheck

00:00:32

failed #645353
cppcheck

00:00:09

failed #641183
cppcheck

00:00:10

 
  Test
passed #641187
build_maximal

00:05:30

passed #641188
build_minimal

00:04:24

canceled #641186
privileged
sanitizer_gcc

 
  Manual
skipped #641189
allowed to fail manual
codequality
skipped #641190
allowed to fail manual
coverage
skipped #641192
allowed to fail manual
installed_tests
skipped #641191
privileged allowed to fail manual
sanitizer_clang
skipped #641193
allowed to fail manual
valgrind
skipped #641196
allowed to fail manual
zeal_11
skipped #641194
allowed to fail manual
zeal_2
skipped #641195
allowed to fail manual
zeal_4