merge: merge with the default upstream branch without argument

"git merge" without specifying any commit is a no-op by default.

A new option merge.defaultupstream can be set to true to cause such an
invocation of the command to merge the upstream branches configured for
the current branch by using their last observed values stored in their
remote tracking branches.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-merge.txt

@@ -11,7 +11,7 @@ SYNOPSIS
 [verse]
 'git merge' [-n] [--stat] [--no-commit] [--squash]
 	[-s <strategy>] [-X <strategy-option>]
-	[--[no-]rerere-autoupdate] [-m <msg>] <commit>...
+	[--[no-]rerere-autoupdate] [-m <msg>] [<commit>...]
 'git merge' <msg> HEAD <commit>...
 'git merge' --abort
 
@@ -95,8 +95,13 @@ commit or stash your changes before running 'git merge'.
 
 <commit>...::
 	Commits, usually other branch heads, to merge into our branch.
-	You need at least one <commit>.  Specifying more than one
-	<commit> obviously means you are trying an Octopus.
+	Specifying more than one commit will create a merge with
+	more than two parents (affectionately called an Octopus merge).
++
+If no commit is given from the command line, and if `merge.defaultToUpstream`
+configuration variable is set, merge the remote tracking branches
+that the current branch is configured to use as its upstream.
+See also the configuration section of this manual page.
 
 
 PRE-MERGE CHECKS

Documentation/merge-config.txt

@@ -6,6 +6,16 @@ merge.conflictstyle::
 	a `>>>>>>>` marker.  An alternate style, "diff3", adds a `|||||||`
 	marker and the original text before the `=======` marker.
 
+merge.defaultToUpstream::
+	If merge is called without any commit argument, merge the upstream
+	branches configured for the current branch by using their last
+	observed values stored in their remote tracking branches.
+	The values of the `branch.<current branch>.merge` that name the
+	branches at the remote named by `branch.<current branch>.remote`
+	are consulted, and then they are mapped via `remote.<remote>.fetch`
+	to their corresponding remote tracking branches, and the tips of
+	these tracking branches are merged.
+
 merge.log::
 	In addition to branch names, populate the log message with at
 	most the specified number of one-line descriptions from the

builtin/merge.c

@@ -25,6 +25,7 @@
 #include "help.h"
 #include "merge-recursive.h"
 #include "resolve-undo.h"
+#include "remote.h"
 
 #define DEFAULT_TWOHEAD (1<<0)
 #define DEFAULT_OCTOPUS (1<<1)
@@ -37,7 +38,7 @@ struct strategy {
 };
 
 static const char * const builtin_merge_usage[] = {
-	"git merge [options] <commit>...",
+	"git merge [options] [<commit>...]",
 	"git merge [options] <msg> HEAD <commit>",
 	"git merge --abort",
 	NULL
@@ -59,6 +60,7 @@ static int option_renormalize;
 static int verbosity;
 static int allow_rerere_auto;
 static int abort_current_merge;
+static int default_to_upstream;
 
 static struct strategy all_strategy[] = {
 	{ "recursive",  DEFAULT_TWOHEAD | NO_TRIVIAL },
@@ -537,6 +539,9 @@ static int git_merge_config(const char *k, const char *v, void *cb)
 		if (is_bool && shortlog_len)
 			shortlog_len = DEFAULT_MERGE_LOG_LEN;
 		return 0;
+	} else if (!strcmp(k, "merge.defaulttoupstream")) {
+		default_to_upstream = git_config_bool(k, v);
+		return 0;
 	}
 	return git_diff_ui_config(k, v, cb);
 }
@@ -912,6 +917,35 @@ static int evaluate_result(void)
 	return cnt;
 }
 
+/*
+ * Pretend as if the user told us to merge with the tracking
+ * branch we have for the upstream of the current branch
+ */
+static int setup_with_upstream(const char ***argv)
+{
+	struct branch *branch = branch_get(NULL);
+	int i;
+	const char **args;
+
+	if (!branch)
+		die("No current branch.");
+	if (!branch->remote)
+		die("No remote for the current branch.");
+	if (!branch->merge_nr)
+		die("No default upstream defined for the current branch.");
+
+	args = xcalloc(branch->merge_nr + 1, sizeof(char *));
+	for (i = 0; i < branch->merge_nr; i++) {
+		if (!branch->merge[i]->dst)
+			die("No remote tracking branch for %s from %s",
+			    branch->merge[i]->src, branch->remote_name);
+		args[i] = branch->merge[i]->dst;
+	}
+	args[i] = NULL;
+	*argv = args;
+	return i;
+}
+
 int cmd_merge(int argc, const char **argv, const char *prefix)
 {
 	unsigned char result_tree[20];
@@ -984,6 +1018,9 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 	if (!allow_fast_forward && fast_forward_only)
 		die("You cannot combine --no-ff with --ff-only.");
 
+	if (!argc && !abort_current_merge && default_to_upstream)
+		argc = setup_with_upstream(&argv);
+
 	if (!argc)
 		usage_with_options(builtin_merge_usage,
 			builtin_merge_options);