GIT_ASKPASS absolute path detection bug on Windows

Hi All,

I believe to have found an issue regarding properly executing the GIT_ASKPASS binary. I'm using Windows Server 2019, with git 2.21.0 installed using cygwin.

## To reproduce:

Assume you have the askpass binary at C:\askpass.bat. In CMD the following commands reproduce the issue:

C:\> set GIT_ASKPASS=C:\askpass.bat C:\> git clone https://<private_repository>.git Cloning into '<private_repository>'... error: cannot run C:\askpass.bat: No such file or directory [... proceeds to interactively ask for username and password ...]

On the other hand, if we change the GIT_ASKPASS environment variable slightly, so that there is a forward slash (/) instead of a backslash (\), things work as expected:

C:\> set GIT_ASKPASS=C:/askpass.bat C:\> git clone https://<private_repository>.git Cloning into '<private_repository>'... [... success ...]

## Some context:

The source of the problem, is that if git doesn't find a forward slash anywhere in the path, it assumes it is not a real path and has to look for the binary using the PATH environment variable. See in prepare_cmd(): https://github.com/git/git/blob/98cedd0233e/run-command.c#L429-L439

You can see that the "cannot run" error message is printed here, just after prepare_cmd() returned -1: https://github.com/git/git/blob/98cedd0233e/run-command.c#L749-L753

I believe this was introduced in late 2018 around git v2.19.2, although I did not actually bisect the issue: https://github.com/git/git/commit/321fd823897#diff-7577a5178f8cdc0f719e580577889f04R401-R415

I hope I'm sharing this bug at the right forum. Please direct me to the proper place if not.

Thank you, Andras