[Fix JENKINS-22706] Warn Windows users if TEMP or TMP value contains embedded space

Some msysgit internal credentials operations fail with a message that
"C:\Program" is not a valid path if the TEMP or TMP directory name
contains an embedded space.

This warning is intended to help the user diagnose and repair their
configuration error.
Merge commit '96f9c8bd9c1229bfe644e0a30356208890a53696' into HEAD