[3/15] refactor(dag): extract `query_stack_commits` to `Dag`

[arxanas]

Stack:

• https://github.com/arxanas/git-branchless/pull/1220
• https://github.com/arxanas/git-branchless/pull/1221
• https://github.com/arxanas/git-branchless/pull/1222
• https://github.com/arxanas/git-branchless/pull/1223
• https://github.com/arxanas/git-branchless/pull/1224
• https://github.com/arxanas/git-branchless/pull/1225
• https://github.com/arxanas/git-branchless/pull/1226
• https://github.com/arxanas/git-branchless/pull/1227
• https://github.com/arxanas/git-branchless/pull/1228
• https://github.com/arxanas/git-branchless/pull/1229
• https://github.com/arxanas/git-branchless/pull/1184
• https://github.com/arxanas/git-branchless/pull/1230
• https://github.com/arxanas/git-branchless/pull/1231
• https://github.com/arxanas/git-branchless/pull/1232
• https://github.com/arxanas/git-branchless/pull/1233

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refactor(dag): extract query_stack_commits to Dag

So that we can use this logic in git-branchless-submit, which wants to be fundamentally aware of commit stacks as part of submitting.