Set min_crossrefs to default to 2, not 0.

[jneem]

In the original bibtex.web, the default value for min_crossrefs is 2, not 0. Thia matches that behaviour.

The bibtex distributed with texlive also defaults to min_crossrefs = 2 (and it also makes it configurable at run-time, with a CLI arg, which this patch doesn't do).

[codecov]

Codecov Report

Merging #534 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #534      +/-   ##
=========================================
+ Coverage    45.5%   45.5%   +<.01%     
=========================================
  Files         137     137              
  Lines       59911   59912       +1     
=========================================
+ Hits        27263   27264       +1     
  Misses      32648   32648

Impacted Files	Coverage Δ
tectonic/bibtex.c	44.06% <100%> (+0.01%)	⬆️

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[pkgw]

Thanks! See #175, #475 for some related discussion.

If I had any actual time to work on this, I'd create a prototype new CLI client with a "multitool" style interface (e.g. tectonic init, tectonic build, tectonic show-cached-file) and a Tectonic.toml file defining the document in which one could set low-level parameters such as min_crossrefs.