hgweb: set sent_headers attr as early as practical While doing Python 3 porting work, I've seen exceptions happen in parts of hgweb we normally assume are robust. It won't hurt anything to set this attribute significantly earlier, so let's do so and save confusing during the porting process. Differential Revision: https://phab.mercurial-scm.org/D1085

hgweb: detect Python 3-era libraries and use modern attribute names Differential Revision: https://phab.mercurial-scm.org/D1084

server: indent block that's about to get conditionalized Differential Revision: https://phab.mercurial-scm.org/D1083

templates: fix missed space between instability labels

templates: introduce labelcset() function in map-cmdline.default as example

templater: load aliases from [templatealias] section in map file This seems sometimes useful as an alias can be a function, but a template fragment can't.

templater: load template fragments from [templates] section in map file This allows us to %include map-cmdline.<style> file in our .hgrc files. The syntax is slightly different as hgrc doesn't support loading an external template file, but map-cmdline files don't use this feature, so the syntax can be considered identical in practice. Unnamed section is remapped for backward compatibility.

config: allow remapping the default section The next patch depends on it. It doesn't make sense that the default section can't be remapped with {'': whatever}.

templater: simplify merge of __base__ dicts by reading it first

phase: add a dedicated pretxnclose-phase hook This new hook mirror the newly introduced 'txnclose-phase' but can abort the transaction.