help: add "man.viewer" config var to use "woman" or "konqueror"

This patch makes it possible to view man pages using other tools
than the "man" program. It also implements support for emacs'
"woman" and konqueror with the man KIO slave to view man pages.

Note that "emacsclient" is used with option "-e" to launch "woman"
on emacs and this works only on versions >= 22.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Tested-by: Xavier Maillard <xma@gnu.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

help.c

@@ -8,6 +8,9 @@
 #include "exec_cmd.h"
 #include "common-cmds.h"
 #include "parse-options.h"
+#include "run-command.h"
+
+static const char *man_viewer;
 
 enum help_format {
 	HELP_FORMAT_MAN,
@@ -50,6 +53,8 @@ static int git_help_config(const char *var, const char *value)
 		help_format = parse_help_format(value);
 		return 0;
 	}
+	if (!strcmp(var, "man.viewer"))
+		return git_config_string(&man_viewer, var, value);
 	return git_default_config(var, value);
 }
 
@@ -345,11 +350,85 @@ static void setup_man_path(void)
 	strbuf_release(&new_path);
 }
 
+static int check_emacsclient_version(void)
+{
+	struct strbuf buffer = STRBUF_INIT;
+	struct child_process ec_process;
+	const char *argv_ec[] = { "emacsclient", "--version", NULL };
+	int version;
+
+	/* emacsclient prints its version number on stderr */
+	memset(&ec_process, 0, sizeof(ec_process));
+	ec_process.argv = argv_ec;
+	ec_process.err = -1;
+	ec_process.stdout_to_stderr = 1;
+	if (start_command(&ec_process)) {
+		fprintf(stderr, "Failed to start emacsclient.\n");
+		return -1;
+	}
+	strbuf_read(&buffer, ec_process.err, 20);
+	close(ec_process.err);
+
+	/*
+	 * Don't bother checking return value, because "emacsclient --version"
+	 * seems to always exits with code 1.
+	 */
+	finish_command(&ec_process);
+
+	if (prefixcmp(buffer.buf, "emacsclient")) {
+		fprintf(stderr, "Failed to parse emacsclient version.\n");
+		strbuf_release(&buffer);
+		return -1;
+	}
+
+	strbuf_remove(&buffer, 0, strlen("emacsclient"));
+	version = atoi(buffer.buf);
+
+	if (version < 22) {
+		fprintf(stderr,
+			"emacsclient version '%d' too old (< 22).\n",
+			version);
+		strbuf_release(&buffer);
+		return -1;
+	}
+
+	strbuf_release(&buffer);
+	return 0;
+}
+
+static void exec_woman_emacs(const char *page)
+{
+	if (!check_emacsclient_version()) {
+		/* This works only with emacsclient version >= 22. */
+		struct strbuf man_page = STRBUF_INIT;
+		strbuf_addf(&man_page, "(woman \"%s\")", page);
+		execlp("emacsclient", "emacsclient", "-e", man_page.buf, NULL);
+	} else
+		execlp("man", "man", page, NULL);
+}
+
+static void exec_man_konqueror(const char *page)
+{
+	const char *display = getenv("DISPLAY");
+	if (display && *display) {
+		struct strbuf man_page = STRBUF_INIT;
+		strbuf_addf(&man_page, "man:%s(1)", page);
+		execlp("kfmclient", "kfmclient", "newTab", man_page.buf, NULL);
+	} else
+		execlp("man", "man", page, NULL);
+}
+
 static void show_man_page(const char *git_cmd)
 {
 	const char *page = cmd_to_page(git_cmd);
 	setup_man_path();
-	execlp("man", "man", page, NULL);
+	if (!man_viewer || !strcmp(man_viewer, "man"))
+		execlp("man", "man", page, NULL);
+	if (!strcmp(man_viewer, "woman"))
+		exec_woman_emacs(page);
+	if (!strcmp(man_viewer, "konqueror"))
+		exec_man_konqueror(page);
+	die("'%s': unsupported man viewer.", man_viewer);
 }
 
 static void show_info_page(const char *git_cmd)