Reuse existing outputs

[stanmart]

tectonic is amazing for generating outputs from scratch, and it is quite fast at that. However, when outputs from older runs already exist, it does not make use of those to speed up the compilation.

In contrast, latexmk has the ability to save some latex runs whenever some output already exists. E.g.,

• If the inputs have not changed, it does not recompile the document.
• Sometime when the inputs only have some small changes, one pdflatex run suffices.

It would be nice if tectonic would have similar features. I guess as a default, the current behavior is better/safer. However, this quasi-incremental build feature would be especially convenient when using with some editor with compile on save enabled.

What do you think?