The php7.0 package in Ubuntu 16.04 splits out the php7.0-xml package, which
provides the utf8_decode function. As such, parts of Arcanist throw
exceptions -- most notably PhutilArgumentSpellingCorrector:
$ arc dieff [2017-01-12 14:57:25] EXCEPTION: (Error) Call to undefined function utf8_decode() at [<phutil>/src/utils/utf8.php:259] arcanist(head=master, ref.master=d6e112aecf93), phutil(head=master, ref.master=ab80dcf99c6b) #0 phutil_utf8_strlen(string) called at [<phutil>/src/parser/argument/PhutilArgumentSpellingCorrector.php:120] #1 PhutilArgumentSpellingCorrector::correctSpelling(string, array) called at [<arcanist>/src/configuration/ArcanistConfiguration.php:148] #2 ArcanistConfiguration::selectWorkflow(string, array, ArcanistConfigurationManager, PhutilConsole) called at [<arcanist>/scripts/arcanist.php:193]
Since spelling correction is a core feature, and working out-of-the-box is
desirable, fall back to implementing phutil_utf8_strlen in terms of
phutil_utf8v if decode_utf8 is not present.
Fixes: T11713.