I would suggest breaking it into 2 batch files. The first one would be to create the environment and the second would just compile the client.
Yeah, i bet everyone who will use it for second time will do it anyway.
Relevant lines for [pulling updates and] re-building bitcoin:
for /F "delims=" %%I in ("%~dp0") do set install_root=%%~fI
set PATH=%install_root%\msysgit\bin;%install_root%\msysgit\mingw\bin;%PATH%
cd bitcoin
::git pull
cd src
bash -i -c 'make -f makefile.mingw INCLUDEPATHS="-I../../boost -I../../db-4.7.25.NC/build_unix -I../../openssl-1.0.0d/include -I../../wxWidgets/lib/gcc_lib/mswu -I../../wxWidgets/include -DBOOST_THREAD_USE_LIB -UUSE_UPNP" LIBPATHS="-L../../boost/stage/lib -L../../db-4.7.25.NC/build_unix -L../../openssl-1.0.0d -L../../wxWidgets/lib/gcc_lib" WXLIBS="-lwxmsw29u_html -lwxmsw29u_core -lwxmsw29u_adv -lwxbase29u -lwxtiff -lwxjpeg -lwxpng -lwxzlib" LIBS="-lboost_system-mgw44-mt-1_48 -lboost_filesystem-mgw44-mt-1_48 -lboost_program_options-mgw44-mt-1_48 -lboost_thread-mgw44-mt-1_48 -ldb_cxx -lssl -lcrypto -l kernel32 -l user32 -l gdi32 -l comdlg32 -l winspool -l winmm -l shell32 -l comctl32 -l ole32 -l oleaut32 -l uuid -l rpcrt4 -l advapi32 -l ws2_32 -l shlwapi"'
strip bitcoin.exe
P.S. Edit: uncomment 'git pull' if you want to update source tree