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)

---
core.gitProxy can also be used for my purpose (I need pushing, not fetching though; the document says for fetching but I guess core.gitProxy always works for pushing), but then the key file is hidden behind the script. And writing a script for this seems a bit overkill.

 Documentation/config.txt |  7 +++++++
 connect.c                | 15 ++++++++++++++-
 2 files changed, 21 insertions(+), 1 deletion(-)

index 58673cf..3bf070c 100644
@@ -443,6 +443,13 @@ specify that no proxy be used for a given domain pattern.
 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 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 is in the same form as
+	'GIT_SSH_COMMAND' environment variable and is overriden 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

index c53f3f1..722dc3f 100644
@@ -658,6 +658,19 @@ static enum protocol parse_connect_url(const char *url_orig, char **ret_host,
 
 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 @@ struct child_process *git_connect(int fd[2], const char *url,
 				return NULL;
 			}
 
-			ssh = getenv("GIT_SSH_COMMAND");
+			ssh = get_ssh_command();
 			if (!ssh) {
 				const char *base;
 				char *ssh_dup;

2.8.2.530.g66e63ce

This patch makes sense to me. The way I have done this in the past is to use ssh's config. So I set my remote in one repo to "foo:repo1.git" and another to "bar:repo2.git", and then:

Host foo
Hostname actual-host.example.com
SomeOption ...

Host bar
Hostname actual-host.example.com
SomeOption ...

(or more likely, most repos are fine with the defaults, and you only need one fake host block for the oddball repo).

But I think your solution is a more direct and less confusing way of accomplishing the same thing.

I think so. I'd write either "same form as GIT_SSH_COMMAND" or "same form as the GIT_SSH_COMMAND environment variable`.

Yes, we've tried to make the docs a bit more consistant, and to document that better in Documentation/CodingGuidelines:

An environment variable must be prefixed with "$" only when referring to its value and not when referring to the variable itself, in this case there is nothing to add except the backticks:
GIT_DIR is specified
$GIT_DIR/hooks/pre-receive

Backticks too:

Literal examples (e.g. use of command-line options, command names,
branch names, configuration and environment variables) must be typeset in monospace (i.e. wrapped with backticks):
--pretty=oneline
git rev-list

Matthieu Moy
http://www-verimag.imag.fr/~moy/

Let's then squash this in.

 Documentation/config.txt | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

index bd32069..fe7c9c4 100644
@@ -444,11 +444,11 @@ 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 then 'git fetch' and 'git push' will
-	use the specified command instead of 'ssh' when they need to
+	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
-	'GIT_SSH_COMMAND' environment variable and is overriden when
-	the environment variable is set.
+	the `GIT_SSH_COMMAND` environment variable and is overriden
+	when the environment variable is set.
 
 core.ignoreStat::
 	If true, Git will avoid using lstat() calls to detect if files have