Makefile: Get git revision correctly on Windows

Fixes a bug that `rustc.exe -v` didn't show git revision hash.
The bug is caused by that `$(wildcard $(CFG_GIT))` requires
space-escaped inputs while `$(CFG_GIT)` is usually
`C:\Program Files (x86)\Git\bin\git.exe`.
auto merge of #9748 : klutzy/rust/print-git-revision, r=alexcrichton

Fixes a bug that `rustc.exe -v` didn't show git revision hash.
The bug is caused by that `$(wildcard $(CFG_GIT))` requires
space-escaped inputs while `$(CFG_GIT)` is usually
`C:\Program Files (x86)\Gitin\git.exe`.