git config credential.helper with absolute path on windows not working properly

Hey guys,

I recently found that specifying absolute paths for credential helper
on windows does not work correctly if the path has spaces or non
alphanumeric characters like parentheses. From what I gather it seems
to do with how args are passed to subprocesses.

example:
git config credential.helper "C:\tmp\a (b)\git-credential-helper.cmd"

Running `git config credential.helper` shows that git got the full path:
C:\tmp\a (b)\git-credential-helper.cmd

However, when running `git push`, for example, git has problems
invoking the credential helper. If the path has no spaces or
parentheses, then everything is fine.

I tried countless ways of escaping the path, but couldn't find a
proper way to pass this to git and make it work. Tried adding more
quotes, slashes, and '^' (escape character for batch). Escaping spaces
with slashes seems to work, but couldn't find a way to escape
characters like '(' and '('.
I think git needs to add some escaping or quotes when running the
credential helper.

This affects commands that inline the credential helper too:
git clone https://path.to.repo.com C:\tmp\default --config
credential.helper="c:\tmp\a (b)\git-credential-gcloud.cmd"

Not sure if this is limited to credential helper or any config in git,
but this is a case I could repro easily.

Tested on Windows 7 x64 with all updates installed, from the command
line, using git version 2.6.0.windows.1. Git is installed to be
available from the command line.


Thanks,
Valentin