py3: use bytes.endswith() instead of bytes[n]

diff: graduate word-diff option from experimental Per 4.6 Sprint notes. I've also made it gated by "formatchanging" since it could change the output if we had an option to highlight words without using colors.

fix: add test case that shows why --whole with --base is useful Differential Revision: https://phab.mercurial-scm.org/D3894

context: raise ProgrammingError on repo['my-tag'] We had an internal extension that I had failed to migrate off of the deprecated API and its "'my-tag' in repo" check just started returning False. It took a while to figure out that that was what was happening. This patch would have helped. Differential Revision: https://phab.mercurial-scm.org/D3895

diffutil: move the module out of utils package mercurial.utils modules inherit the property of the mercurial.util, which is no dependency on ui, repo, ctx, etc. As the diffutil module seems to reside in the scmutil layer, it's probably better to not put it under the utils package.

diffutil: remove diffopts() in favor of diffallopts() patch.diffopts() exists only for backward compatibility. We don't need it in new module.

obsutil: use public interface to access to repo.ui

hgweb: pass ui to diffstatgen() explicitly

synthrepo: simply use the ui passed as a function argument

templatekw: obtain ui directly from the template context

context: remove unneeded alias of diffopts

match: remove ctx argument from code path down to _buildmatch() 'ctx' was there only for filesets.