userdiff: add diff.<driver>.process config

Add a new per-driver configuration key that specifies the command
for a long-running diff process.

The name follows filter.<driver>.process: a long-running subprocess
that stays alive across files within a single git invocation.

Signed-off-by: Michael Montalbo <mmontalbo@gmail.com>

Author: mmontalbo

userdiff.c

@@ -499,6 +499,13 @@ int userdiff_config(const char *k, const char *v)
 		drv->algorithm = drv->algorithm_owned;
 		return ret;
 	}
+	if (!strcmp(type, "process")) {
+		int ret;
+		FREE_AND_NULL(drv->process_owned);
+		ret = git_config_string(&drv->process_owned, k, v);
+		drv->process = drv->process_owned;
+		return ret;
+	}
 
 	return 0;
 }

userdiff.h

@@ -31,6 +31,8 @@ struct userdiff_driver {
 	char *textconv_owned;
 	struct notes_cache *textconv_cache;
 	int textconv_want_cache;
+	const char *process;
+	char *process_owned;
 };
 enum userdiff_driver_type {
 	USERDIFF_DRIVER_TYPE_BUILTIN = 1<<0,