msvc: add a compile-time flag to allow detailed heap debugging

MS Visual C comes with a few neat features we can use to analyze the
heap consumption (i.e. leaks, max memory, etc).

With this patch, we introduce support via the build-time flag
`USE_MSVC_CRTDBG`.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/mingw.c

@@ -2411,6 +2411,12 @@ int wmain(int argc, const wchar_t **wargv)
 
 	trace2_initialize_clock();
 
+#ifdef _MSC_VER
+#ifdef USE_MSVC_CRTDBG
+	_CrtSetDbgFlag(_CRTDBG_ALLOC_MEM_DF | _CRTDBG_LEAK_CHECK_DF);
+#endif
+#endif
+
 	maybe_redirect_std_handles();
 
 	/* determine size of argv and environ conversion buffer */

config.mak.uname

@@ -448,6 +448,10 @@ else
 endif
 	BASIC_CFLAGS += $(sdk_libs) $(msvc_libs)
 
+ifneq ($(USE_MSVC_CRTDBG),)
+	# Optionally enable memory leak reporting.
+	BASIC_CFLAGS += -DUSE_MSVC_CRTDBG
+endif
 	BASIC_CFLAGS += -DPROTECT_NTFS_DEFAULT=1
 	# Always give "-Zi" to the compiler and "-debug" to linker (even in
 	# release mode) to force a PDB to be generated (like RelWithDebInfo).

git-compat-util.h

@@ -1,6 +1,15 @@
 #ifndef GIT_COMPAT_UTIL_H
 #define GIT_COMPAT_UTIL_H
 
+#ifdef USE_MSVC_CRTDBG
+/*
+ * For these to work they must appear very early in each
+ * file -- before most of the standard header files.
+ */
+#include <stdlib.h>
+#include <crtdbg.h>
+#endif
+
 #define _FILE_OFFSET_BITS 64