cmake: install headless-git. (#4338)

Even if CMake is not the canonical way to build Git for Windows, but
CMake support merely exists in Git to support building Git for Windows
using Visual Studio, we should include `headless-git` in such a scenario
when installing the binaries to a given location.

contrib/buildsystems/CMakeLists.txt

@@ -740,6 +740,7 @@ if(WIN32)
 	endif()
 
 	add_executable(headless-git ${CMAKE_SOURCE_DIR}/compat/win32/headless.c)
+	list(APPEND PROGRAMS_BUILT headless-git)
 	if(CMAKE_C_COMPILER_ID STREQUAL "GNU" OR CMAKE_C_COMPILER_ID STREQUAL "Clang")
 		target_link_options(headless-git PUBLIC -municode -Wl,-subsystem,windows)
 	elseif(CMAKE_C_COMPILER_ID STREQUAL "MSVC")
@@ -919,7 +920,7 @@ list(TRANSFORM git_perl_scripts PREPEND "${CMAKE_BINARY_DIR}/")
 
 #install
 foreach(program ${PROGRAMS_BUILT})
-if(program MATCHES "^(git|git-shell|scalar)$")
+if(program MATCHES "^(git|git-shell|headless-git|scalar)$")
 install(TARGETS ${program}
 	RUNTIME DESTINATION bin)
 else()