git_connect: set ssh shell command in GIT_SSH_COMMAND

It may be impractical to install a wrapper script for GIT_SSH
when additional parameters need to be passed. Provide an alternative
way of specifying a shell command to be run, including command line
arguments, by means of the GIT_SSH_COMMAND environment variable,
which behaves like GIT_SSH but is passed to the shell.

The special circuitry to modify parameters in the case of using
PuTTY's plink/tortoiseplink is activated only when using GIT_SSH;
in the case of using GIT_SSH_COMMAND, it is deliberately left up to
the user to make any required parameters adaptation before calling
the underlying ssh implementation.

Signed-off-by: Thomas Quinot <thomas@quinot.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: quinot

Documentation/git.txt

@@ -876,19 +876,21 @@ other
 	and the `core.editor` option in linkgit:git-config[1].
 
 'GIT_SSH'::
-	If this environment variable is set then 'git fetch'
-	and 'git push' will use this command instead
-	of 'ssh' when they need to connect to a remote system.
-	The '$GIT_SSH' command will be given exactly two or
-	four arguments: the 'username@host' (or just 'host')
-	from the URL and the shell command to execute on that
-	remote system, optionally preceded by '-p' (literally) and
-	the 'port' from the URL when it specifies something other
-	than the default SSH port.
+'GIT_SSH_COMMAND'::
+	If either of these environment variables is set then 'git fetch'
+	and 'git push' will use the specified command instead of 'ssh'
+	when they need to connect to a remote system.
+	The command will be given exactly two or four arguments: the
+	'username@host' (or just 'host') from the URL and the shell
+	command to execute on that remote system, optionally preceded by
+	'-p' (literally) and the 'port' from the URL when it specifies
+	something other than the default SSH port.
 +
-To pass options to the program that you want to list in GIT_SSH
-you will need to wrap the program and options into a shell script,
-then set GIT_SSH to refer to the shell script.
+`$GIT_SSH_COMMAND` takes precedence over `$GIT_SSH`, and is interpreted
+by the shell, which allows additional arguments to be included.
+`$GIT_SSH` on the other hand must be just the path to a program
+(which can be a wrapper shell script, if additional arguments are
+needed).
 +
 Usually it is easier to configure any desired options through your
 personal `.ssh/config` file.  Please consult your ssh documentation

connect.c

@@ -700,14 +700,23 @@ struct child_process *git_connect(int fd[2], const char *url,
 
 		conn->in = conn->out = -1;
 		if (protocol == PROTO_SSH) {
-			const char *ssh = getenv("GIT_SSH");
-			int putty = ssh && strcasestr(ssh, "plink");
+			const char *ssh;
+			int putty;
 			char *ssh_host = hostandport;
 			const char *port = NULL;
 			get_host_and_port(&ssh_host, &port);
 			port = get_port_numeric(port);
 
-			if (!ssh) ssh = "ssh";
+			ssh = getenv("GIT_SSH_COMMAND");
+			if (ssh) {
+				conn->use_shell = 1;
+				putty = 0;
+			} else {
+				ssh = getenv("GIT_SSH");
+				if (!ssh)
+					ssh = "ssh";
+				putty = !!strcasestr(ssh, "plink");
+			}
 
 			argv_array_push(&conn->args, ssh);
 			if (putty && !strcasestr(ssh, "tortoiseplink"))