Sum hoc only for single-parent commits in total_commits_pushed

[Thayorns]

The history query in total_commits_pushed already fetches parents.totalCount for every commit but never used it, so a merge commit's additions and deletions were added on top of the additions and deletions of every commit it brought in through history, inflating the hoc count by roughly the size of each merged branch.

This filters the sum to commits with exactly one parent, using the field the query already pays for, and locks the fixed behavior down with a test where a page of pure merge commits contributes nothing to hoc.

Closes #659

[Thayorns]

@yegor256 could you take a look, happy to clarify anything about the fix.

[yegor256]

This PR now has merge conflicts and needs a rebase before it can go in. Assigning to @yegor256 for a look.

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Generated by Claude Code

[Thayorns]

@yegor256 take a look please