How to sort (by whatever key) BibTex entries in `.bib` file?

How about bibsort?

NAME
     bibsort - sort a BibTeX bibliography file

SYNOPSIS
     bibsort [optional sort(1) switches] < infile >outfile

DESCRIPTION
     bibsort filters a BibTeX bibliography, or bibliography frag-
     ment,  on  its standard input, printing on standard output a
     sorted bibliography.

It's a shell script wrapping nawk (and tr, sort and grep) and includes two warnings you might have to pay attention to (see the source).

(Edit There're also a lot of bibtex-related Perl modules...)

Edit2 I just recognized you'd like to sort for any key, while bibsort apparently sorts by the bibtex tags only -- but maybe its source (it's not too long) is still helpful...?

You can use bibtool (-s option - sort according to the reference key) with the git post-commit hook.

The reference manager jabref seems to have such an option in the preferences (see Sort order section in File page). It also has many interesting features: