use sh as SHELL for Cygwin/Git-Bash

[gucong3000]

Related: npm/npm#18580 npm/npm#20487

[zkat]

cc @iarna I believe this would be a breaking change. Usually, we wouldn't support cygwin, but we do try to support git bash/msysgit.

[iarna]

@zkat This is definitely a bug in npm-lifecycle, imo. We detect msys elsewhere, but not here. As npm/npm#20487 suggests, I think the right solution is to pass in a value computed by npm. But that option should definitely have a default for non-npm and legacy uses.

[isaacs]

I'm finding it challenging to follow the reasoning for this. It looks like it's doing the following:

• Look up the install location of git in the registry
• Join that base dir to a bunch of other directories.
• Make sure all of those are in the env.PATH
• Join the cmd to that base dir as well.
• Whenever cmd starts with /, join it to that as well.
• But only when on a windows system, where the caller has set the isWindowsShell config.

I'm definitely going to need tests to land this.