exec_cmd: provide a new-style RUNTIME_PREFIX helper for Windows

The RUNTIME_PREFIX feature comes from Git for Windows, but it was
enhanced to allow support for other platforms. While changing the
original idea, the concept was also improved by not forcing argv[0] to
be adjusted.

Let's allow the same for Windows by implementing a helper just as for
the other platforms.

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

Makefile

@@ -453,6 +453,10 @@ all::
 # When using RUNTIME_PREFIX, define HAVE_NS_GET_EXECUTABLE_PATH if your platform
 # supports calling _NSGetExecutablePath to retrieve the path of the running
 # executable.
+#
+# When using RUNTIME_PREFIX, define HAVE_WPGMPTR if your platform offers
+# the global variable _wpgmptr containing the absolute path of the current
+# executable (this is the case on Windows).
 
 GIT-VERSION-FILE: FORCE
 	@$(SHELL_PATH) ./GIT-VERSION-GEN
@@ -1693,6 +1697,10 @@ ifdef HAVE_NS_GET_EXECUTABLE_PATH
 	BASIC_CFLAGS += -DHAVE_NS_GET_EXECUTABLE_PATH
 endif
 
+ifdef HAVE_WPGMPTR
+	BASIC_CFLAGS += -DHAVE_WPGMPTR
+endif
+
 ifeq ($(TCLTK_PATH),)
 NO_TCLTK = NoThanks
 endif

exec_cmd.c

@@ -144,6 +144,24 @@ static int git_get_exec_path_darwin(struct strbuf *buf)
 }
 #endif /* HAVE_NS_GET_EXECUTABLE_PATH */
 
+#ifdef HAVE_WPGMPTR
+/*
+ * Resolves the executable path by using the global variable _wpgmptr.
+ *
+ * Returns 0 on success, -1 on failure.
+ */
+static int git_get_exec_path_wpgmptr(struct strbuf *buf)
+{
+	int len = wcslen(_wpgmptr) * 3 + 1;
+	strbuf_grow(buf, len);
+	len = xwcstoutf(buf->buf, _wpgmptr, len);
+	if (len < 0)
+		return -1;
+	buf->len += len;
+	return 0;
+}
+#endif /* HAVE_WPGMPTR */
+
 /*
  * Resolves the absolute path of the current executable.
  *
@@ -178,6 +196,10 @@ static int git_get_exec_path(struct strbuf *buf, const char *argv0)
 		git_get_exec_path_procfs(buf) &&
 #endif /* PROCFS_EXECUTABLE_PATH */
 
+#ifdef HAVE_WPGMPTR
+		git_get_exec_path_wpgmptr(buf) &&
+#endif /* HAVE_WPGMPTR */
+
 		git_get_exec_path_from_argv0(buf, argv0)) {
 		return -1;
 	}