Parse credential.helper command line using shell-words

[arlosi]

Since sh is not available by default on Windows, git2-rs currently splits the credential
helper string by whitespace, which attempts to execute an invalid command line in the case
of the default credential helper on Windows.

The default git credential helper on Windows (Git Credential Manager Core) is set up as
C:/Program\ Files\ \(x86\)/Git\ Credential\ Manager\ Core/git-credential-manager-core.exe

This change resolves the issue by parsing the command line using shell_words,
which follows the unix-style parsing rules used by sh. shell-words is a small crate that
does not have any additional dependencies.

[alexcrichton]

Thanks for this! I'm not sure if this is the right implementation for Windows, though. Do you know what git itself does in situations like this? Could/should we perhaps delegate to cmd instead of sh if it fails?

[arlosi]

I think it's correct to always interpret it as a Unix shell command. git for Windows includes sh, and git always runs the credential helper command through sh.

Here's a screenshot from Process Explorer when the credential helper is prompting for credentials:

Running the command through cmd will fail, since the backslashes will be interpreted as directory separators instead of escapes. Using cmd would work if the user actually configured their credential.helper to use backslash separated paths, but that would break git - so it seems unlikely.

Also, the current defaults situation isn't quite as bad as I originally thought. When GCMC is installed bundled with git, its set up as manager-core, rather than the full path. Only users who installed GCMC separately will be broken (like me).