mdiff: extract a checknonewline inner function in unidiff()

mdiff: distinguish diff headers from hunks in unidiff() Let unidiff return the list of headers it produces (lines '--- <original>' and '+++ <new>') apart from diff hunks. In patch.diff(), we combine headers generated there (not specific to unified format) with those from unidiff(). By returning a list of header lines, we do not append new lines in datetag inner function of unidiff() so that all header lines are '\n'.join-ed in a similar way.

test: end printed diff "hunks" with an empty string in test-context.py So that the resulting diff is correct and does not include a spurious empty line between lines "diff --git a/foo b/foo" and "--- a/foo".

mdiff: let _unidiff yield hunks as (<range information>, <hunk lines>) Now _unidiff yields each hunk lines packed into a tuple with the "range information" `(s1, l1, s2, l2)` that is used to build the typical hunk header '@@ -s1,l1 +s2,l2 @@'. This will be used to make it possible to filter diff hunks based on this range information. The new "range information" is ignored in unidiff() (only caller of _unidiff) for now.

mdiff: turn the comment above _unidiff into a docstring

mdiff: compute newlines-splitted texts within _unidiff There is no reason to compute splitted texts l1, l2 in unidiff() before calling _unidiff as they are only used with the latter function.

dispatch: add pagination of two more help cases I missed these in the last round anf Yuya spotted them in review. Thanks!

help: avoid mutating passed-in `keep` list in `formattedhelp`

extensions: use [0:1] slice on config path instead of [0] This behaves the same in Python 2 and Python 3, even though the path is a bytes.

extensions: use inspect module instead of func_code.co_argcount Fixes the extsetup argspec check on Python 3.