connect: read $GIT_SSH_COMMAND from config file

Similar to $GIT_ASKPASS or $GIT_PROXY_COMMAND, we also read from
config file first then fall back to $GIT_SSH_COMMAND.

This is useful for selecting different private keys targetting the
same host (e.g. github)

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

diff --git a/Documentation/config.txt b/Documentation/config.txt
index 2e1b2e4..4a870d8 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -443,6 +443,13 @@
 This is useful for excluding servers inside a firewall from
 proxy use, while defaulting to a common proxy for external domains.
 
+core.sshCommand::
+	If this variable is set, `git fetch` and `git push` will
+	use the specified command instead of `ssh` when they need to
+	connect to a remote system. The command is in the same form as
+	the `GIT_SSH_COMMAND` environment variable and is overridden
+	when the environment variable is set.
+
 core.ignoreStat::
 	If true, Git will avoid using lstat() calls to detect if files have
 	changed by setting the "assume-unchanged" bit for those tracked files

connect.c

diff --git a/connect.c b/connect.c
index c53f3f1..722dc3f 100644
--- a/connect.c
+++ b/connect.c
@@ -658,6 +658,19 @@
 
 static struct child_process no_fork = CHILD_PROCESS_INIT;
 
+static const char *get_ssh_command(void)
+{
+	const char *ssh;
+
+	if ((ssh = getenv("GIT_SSH_COMMAND")))
+		return ssh;
+
+	if (!git_config_get_string_const("core.sshcommand", &ssh))
+		return ssh;
+
+	return NULL;
+}
+
 /*
  * This returns a dummy child_process if the transport protocol does not
  * need fork(2), or a struct child_process object if it does.  Once done,
@@ -758,7 +771,7 @@
 				return NULL;
 			}
 
-			ssh = getenv("GIT_SSH_COMMAND");
+			ssh = get_ssh_command();
 			if (!ssh) {
 				const char *base;
 				char *ssh_dup;