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.c

@@ -1017,3 +1017,25 @@ sig_handler_t mingw_signal(int sig, sig_handler_t handler)
 	timer_fn = handler;
 	return old;
 }
+
+static const char *make_backslash_path(const char *path)
+{
+	static char buf[PATH_MAX + 1];
+	char *c;
+
+	if (strlcpy(buf, path, PATH_MAX) >= PATH_MAX)
+		die("Too long path: %.*s", 60, path);
+
+	for (c = buf; *c; c++) {
+		if (*c == '/')
+			*c = '\\';
+	}
+	return buf;
+}
+
+void mingw_open_html(const char *unixpath)
+{
+	const char *htmlpath = make_backslash_path(unixpath);
+	printf("Launching default browser to display HTML ...\n");
+	ShellExecute(NULL, "open", htmlpath, NULL, "\\", 0);
+}

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
  */

help.c

@@ -644,14 +644,26 @@ static void get_html_page_path(struct strbuf *page_path, const char *page)
 	strbuf_addf(page_path, "%s/%s.html", html_path, page);
 }
 
+/*
+ * If open_html is not defined in a platform-specific way (see for
+ * example compat/mingw.h), we use the script web--browse to display
+ * HTML.
+ */
+#ifndef open_html
+void open_html(const char *path)
+{
+	execl_git_cmd("web--browse", "-c", "help.browser", path, NULL);
+}
+#endif
+
 static void show_html_page(const char *git_cmd)
 {
 	const char *page = cmd_to_page(git_cmd);
 	struct strbuf page_path; /* it leaks but we exec bellow */
 
 	get_html_page_path(&page_path, page);
 
-	execl_git_cmd("web--browse", "-c", "help.browser", page_path.buf, NULL);
+	open_html(page_path.buf);
 }
 
 void help_unknown_cmd(const char *cmd)