msvc: fix make MSVC=1 install

We used to install into $HOME/bin/, which wreaks havoc with installed
versions of Git for Windows (because $HOME/bin is *prepended* to the
PATH, hence would override `git.exe` in Git Bash).

Let's align the MSVC case with the non-MSVC case and install into
/mingw64/bin/ (or /mingw32/bin/ in 32-bit Git for Windows SDKs) instead.

Noticed by Derrick Stolee.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

config.mak.uname

@@ -342,6 +342,12 @@ endif
 ifeq ($(uname_S),Windows)
 	GIT_VERSION := $(GIT_VERSION).MSVC
 	pathsep = ;
+	# Assume that this is built in Git for Windows' SDK
+	ifeq (MINGW32,$(MSYSTEM))
+		prefix = /mingw32
+	else
+		prefix = /mingw64
+	endif
 	# Prepend MSVC 64-bit tool-chain to PATH.
 	#
 	# A regular Git Bash *does not* have cl.exe in its $PATH. As there is a