rebase: make the backend configurable via config setting

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/rebase.txt

@@ -5,6 +5,14 @@ rebase.useBuiltin::
 	is always used. Setting this will emit a warning, to alert any
 	remaining users that setting this now does nothing.
 
+rebase.backend::
+	Default backend to use for rebasing.  Possible choices are
+	'am' or 'merge' (note that the merge backend is sometimes also
+	refered to as the interactive backend or the interactive
+	machinery elsewhere in the docs).  Also, in the future, if the
+	merge backend gains all remaining capabilities of the am
+	backend, this setting may become unused.
+
 rebase.stat::
 	Whether to show a diffstat of what changed upstream since the last
 	rebase. False by default.

builtin/rebase.c

@@ -60,6 +60,7 @@ enum empty_type {
 struct rebase_options {
 	enum rebase_type type;
 	enum empty_type empty;
+	const char *default_backend;
 	const char *state_dir;
 	struct commit *upstream;
 	const char *upstream_name;
@@ -100,6 +101,7 @@ struct rebase_options {
 #define REBASE_OPTIONS_INIT {			  	\
 		.type = REBASE_UNSPECIFIED,	  	\
 		.empty = EMPTY_UNSPECIFIED,	  	\
+		.default_backend = "am",	  	\
 		.flags = REBASE_NO_QUIET, 		\
 		.git_am_opts = ARGV_ARRAY_INIT,		\
 		.git_format_patch_opt = STRBUF_INIT	\
@@ -1272,6 +1274,10 @@ static int rebase_config(const char *var, const char *value, void *data)
 		return 0;
 	}
 
+	if (!strcmp(var, "rebase.backend")) {
+		return git_config_string(&opts->default_backend, var, value);
+	}
+
 	return git_default_config(var, value, data);
 }
 
@@ -1900,9 +1906,23 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 			if (strcmp(options.git_am_opts.argv[i], "-q"))
 				break;
 
-		if (is_interactive(&options) && i >= 0)
-			die(_("cannot combine am options with either "
-			      "interactive or merge options"));
+		if (i >= 0) {
+			if (is_interactive(&options))
+				die(_("cannot combine am options with either "
+				      "interactive or merge options"));
+			else
+				options.type = REBASE_AM;
+		}
+	}
+
+	if (options.type == REBASE_UNSPECIFIED) {
+		if (!strcmp(options.default_backend, "merge"))
+			options.type = REBASE_MERGE;
+		else if (!strcmp(options.default_backend, "am"))
+			options.type = REBASE_AM;
+		else
+			die(_("Unknown rebase backend: %s"),
+			    options.default_backend);
 	}
 
 	switch (options.type) {
@@ -1915,10 +1935,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 		options.state_dir = apply_dir();
 		break;
 	default:
-		/* the default rebase backend is `--am` */
-		options.type = REBASE_AM;
-		options.state_dir = apply_dir();
-		break;
+		BUG("options.type was just set above; should be unreachable.");
 	}
 
 	if (options.empty == EMPTY_UNSPECIFIED) {