hgweb: start enumerate at 1 in webutil.diffs's inner function prettyprintlines

patch: add a diffhunks function yielding (diffheaders, hunks) trydiff function now yield (header, hunks) tuple that are processed by diffhunks(). Then diff() is a wrapper around diffhunks().

mdiff: let unidiff return (diffheader, hunks) This will be used to make it possible to filter diff hunks based on this range information. Now unidiff returns a 'hunks' generator that yield tuple (hunkrange, hunklines) coming from _unidiff() with 'newline at end of file' processing.

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.