Allow debugging unsafe directories' ownership

When Git refuses to use an existing repository because it is owned by
someone else than the current user, it can be a bit tricky on Windows to
figure out what is going on.

Let's help with that by offering some more information via the
environment variable `GIT_TEST_DEBUG_UNSAFE_DIRECTORIES`.

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

Documentation/config/safe.txt

@@ -40,3 +40,9 @@ which id the original user has.
 If that is not what you would prefer and want git to only trust
 repositories that are owned by root instead, then you can remove
 the `SUDO_UID` variable from root's environment before invoking git.
++
+Due to the permission model on Windows where ACLs are used instead of
+Unix' simpler permission model, it can be a bit tricky to figure out why
+a directory is considered unsafe. To help with this, Git will provide
+more detailed information when the environment variable
+`GIT_TEST_DEBUG_UNSAFE_DIRECTORIES` is set to `true`.

compat/mingw.c

@@ -1,6 +1,7 @@
 #include "../git-compat-util.h"
 #include "win32.h"
 #include <aclapi.h>
+#include <sddl.h>
 #include <conio.h>
 #include <wchar.h>
 #include "../strbuf.h"
@@ -2720,6 +2721,26 @@ int is_path_owned_by_current_sid(const char *path)
 		    IsValidSid(current_user_sid) &&
 		    EqualSid(sid, current_user_sid))
 			result = 1;
+		else if (git_env_bool("GIT_TEST_DEBUG_UNSAFE_DIRECTORIES", 0)) {
+			LPSTR str1, str2, to_free1 = NULL, to_free2 = NULL;
+
+			if (ConvertSidToStringSidA(sid, &str1))
+				to_free1 = str1;
+			else
+				str1 = "(inconvertible)";
+
+			if (!current_user_sid)
+				str2 = "(none)";
+			else if (!IsValidSid(current_user_sid))
+				str2 = "(invalid)";
+			else if (ConvertSidToStringSidA(current_user_sid, &str2))
+				to_free2 = str2;
+			else
+				str2 = "(inconvertible)";
+			warning("'%s' is owned by:\n\t'%s'\nbut the current user is:\n\t'%s'", path, str1, str2);
+			LocalFree(to_free1);
+			LocalFree(to_free2);
+		}
 	}
 
 	/*