ExternalProject: Fix git.cmd version detection

When GIT_EXECUTABLE points at ".../Git/cmd/git.cmd" in an msysGit
installation we previously failed to detect the version number in a
subtle case.  The "git.cmd" assumes 'chcp' is in PATH.  It is typically
available at "C:\Windows\System32\chcp.com".  On 64-bit Windows the File
System Redirector maps this location to "C:\Windows\SysWOW64\chcp.com"
for 32-bit processes.  However, some Windows installations fail to
provide chcp.com at this path.  Whenever git.cmd runs in a 32-bit
command shell, as it does under a 32-bit CMake binary, it reports

 'chcp' is not recognized as an internal or external command,
  operable program or batch file.

on stderr.  Capture stderr separately so it does not affect parsing
of the version number.

See also msysGit issue 358:

  http://code.google.com/p/msysgit/issues/detail?id=358

Note that FindGit prefers "git.cmd" over "git.exe" because it sets up
the proper HOME environment variable necessary for Git ssh connections
to work the same as they do from the Git bash prompt.

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