Expand environment variables used in stowrc, as requested in
https://savannah.gnu.org/bugs/?41826
This is achieved by creating a new function expand_environment() that
replaces any substring of the form '$VAR' or '${VAR}' with contents of
environment variable $VAR. Literal '$' can be given by '\$'.
N.B. The function is only applied to the --target and --dir options,
and only for options specified in .stowrc; cli options are left
untouched.
Undefined variables are expanded to the empty string, as they would be
in normal shell parameter expansion.
Unit tests added accordingly:
- Test expand_environment():
* Expand $HOME
* Expand ${HOME}
* Expand ${WITH SPACE}
* Expand '\$HOME'. Expected is '$HOME'
* Expand ${UNDEFINED}. Expected is ''.
- Test that it's applied to the correct options.
- Test that CLI options are not expanded.
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