templatekw: introduce obsfate keyword
Introduce an obsfate printer that uses all helpers functions defined in
obsutil to get all the obsfate-related data and format a string according to
the current format in test-obsmarker-template.t.
Then, introduce an obsfate templatekw that uses the obsfateprinter to return a
list of strings.
The goal is not to replace existing obsfate template functions but to propose
a default, good-enough and easily usable obsfate definition for end-users that
don't want to customize it. Such output would ultimately get included in the
default log output.
Here are some output examples for a commit amended:
rewritten using amend as 5:
a9b1f8652753 by test (at 1970-01-01 00:00 +0000)
Next patches will make the output dependent on the verbosity.
Exemple of use-cases:
For having the obsfate on a single-line between brackets:
{if(obsfate, " [{join(obsfate, "; ")}]")}
For having the obsfate in several lines:
{if(obsfate, "{obsfate % " Obsfate: {fate}\n"}")}
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