Refer to discussion on D21677#275541
Refs D21681 (arcanist changes)
Phabricator has several uses of the `--debug` flag being used with Mercurial. Use of this flag causes additional output to be added which Phabricator needs, however the behavior of `--debug` is not guaranteed to be stable, and in newer versions of Mercurial there have been additional output that has caused Phabricator to choke on parsing the output. This change removes the use of `--debug` in favor of using `--template` with the `hg log` or `hg annotate` commands in combination with the `{p1.node}` or `{p2.node}` template format.
The use of `{p1node}` format in templates was added in [[ https://www.mercurial-scm.org/wiki/WhatsNew/Archive#Mercurial_2.4_.282012-11-01.29 | Mercurial 2.4 (2012) ]]. This format was deprecated in [[ https://www.mercurial-scm.org/wiki/WhatsNew#Mercurial_4.9_.282019-02-01.29 | Mercurial 4.9 ]] in favor of using `{p1.node}` format which is unclear when this new format was added (presumably earlier than Mercurial 4.9).
The use of `--template` with `hg annotate` is only officially supported in [[ https://www.mercurial-scm.org/wiki/Release4.6 | Mercurial 4.6 (2018) ]], though does appear to work in 4.5 but is not documented.
Because of the version requirements, this updates the required version of Mercurial to be 4.6 or greater.