svnwrap: add runtime override for choice of Subversion bindings By setting the `HGSUBVERSION_BINDINGS environment variable to either `SWIG' or `Subvertpy', the choice of bindings can be forced at runtime. (For ease of use, the comparison is case-insensitive.) Examples: % HGSUBVERSION_BINDINGS=swig hg version --svn Mercurial Distributed SCM (version 1.6+172-b25e1ced9861) ... hgsubversion: 1.1.2+43-276742da2d85 Subversion: 1.6.12 bindings: SWIG % HGSUBVERSION_BINDINGS=subvertpy hg version --svn Mercurial Distributed SCM (version 1.6+172-b25e1ced9861) ... hgsubversion: 1.1.2+43-276742da2d85 Subversion: 1.6.12 bindings: Subvertpy 0.7.4 % HGSUBVERSION_BINDINGS=none hg version --svn Mercurial Distributed SCM (version 1.6+172-b25e1ced9861) ... abort: cannot use hgsubversion; bindings disabled using HGSUBVERSION_BINDINGS!