Support --output=commands flag for aquery

[pcc]

This output format prints a list of build commands with one command per line, similar to ninja -t commands. This is frequently useful when debugging build issues.

Fixes #22389.

[joeleba]

Thanks for your contribution!

[meisterT]

Do we print the lines in a particular order? I see that ninja does: "given a list of targets, print a list of commands which, if executed in order, may be used to rebuild those targets, assuming that all output files are out of date."

[pcc]

No, it's just the iteration order used by bazel aquery which is normally unordered according to https://bazel.build/query/language#graph-order . That being said I normally use it with deps which does have an order but https://bazel.build/query/language#deps just says "The resulting graph is ordered according to the dependency relation". Empirically, it seems to be a pre-order traversal (nodes appear before dependencies, i.e. the opposite of Ninja's order).

[meisterT]

Perhaps include a statement in the docs, saying something like "Do not rely on the order of output."

[pcc]

Perhaps include a statement in the docs, saying something like "Do not rely on the order of output."

Done.