web: a diff cut at 4 MiB says so, and the commit list says how many it hides !215

Diff and ShowPatch cut the patch at the byte limit mid-line and reported nothing, so the last hunk rendered wrong and the diffstat under-reported. Both now cut on a line boundary and return whether they did. The commit and merge request pages show a notice and the diffstat says its counts are partial; mr diff and repo commit say so on stderr. The merge request's commit list is capped at 100 and now says "first 100 of N; the rest are in the branch".

Stacked on !214; no code dependency.

Closes #117