help (Windows): Display HTML in default browser using Windows' shell API

The system's default browser for displaying HTML help pages is now used
directly on Windows, instead of launching git-web--browser, which
requires a Unix shell.  Avoiding MSYS' bash when possible is good
because it avoids potential path translation issues.  In this case it is
not too hard to avoid launching a shell, so let's avoid it.

The Windows-specific code is implemented in compat/mingw.c to avoid
platform-specific code in the main code base.  On Windows, open_html is
provided as a define.  If open_html is not defined, git-web--browse is
used.  This approach avoids platform-specific ifdefs by using
per-function ifdefs.  The "ifndef open_html" together with the
introductory comment should sufficiently warn developers, so that they
hopefully will not break this mechanism.

Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/mingw.h

@@ -202,6 +202,9 @@ sig_handler_t mingw_signal(int sig, sig_handler_t handler);
 #define PATH_SEP ';'
 #define PRIuMAX "I64u"
 
+void mingw_open_html(const char *path);
+#define open_html mingw_open_html
+
 /*
  * helpers
  */