scmutil: extract helper functions that returns human-readable change id
We do "'%d:%s' % (ctx...)" at several places, so let's formalize it. A low-
level function, formatrevnode(ui, rev, node), is extracted so we can pass
a manifest rev/node pair.
Note that hex() for manifest output can be replaced with hexfunc() because
it is printed only when debugflag is set.
i18n/de.po is updated so test-log.t passes with no error.
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