merge: allow reading the merge commit message from a file

This is consistent with `git commit` which, like `git merge`, supports
passing the commit message via `-m <msg>` and, unlike `git merge` before
this patch, via `-F <file>`.

It is useful to allow this for scripted use, or for the upcoming patch
to allow (re-)creating octopus merges in `git rebase --rebase-merges`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-merge.txt

@@ -12,7 +12,7 @@ SYNOPSIS
 'git merge' [-n] [--stat] [--no-commit] [--squash] [--[no-]edit]
 	[-s <strategy>] [-X <strategy-option>] [-S[<keyid>]]
 	[--[no-]allow-unrelated-histories]
-	[--[no-]rerere-autoupdate] [-m <msg>] [<commit>...]
+	[--[no-]rerere-autoupdate] [-m <msg>] [-F <file>] [<commit>...]
 'git merge' --abort
 'git merge' --continue
 
@@ -75,6 +75,14 @@ The 'git fmt-merge-msg' command can be
 used to give a good default for automated 'git merge'
 invocations. The automated message can include the branch description.
 
+-F <file>::
+--file=<file>::
+	Read the commit message to be used for the merge commit (in
+	case one is created).
++
+If `--log` is specified, a shortlog of the commits being merged
+will be appended to the specified message.
+
 --[no-]rerere-autoupdate::
 	Allow the rerere mechanism to update the index with the
 	result of auto-conflict resolution if possible.

builtin/merge.c

@@ -111,6 +111,35 @@ static int option_parse_message(const struct option *opt,
 	return 0;
 }
 
+static int option_read_message(struct parse_opt_ctx_t *ctx,
+			       const struct option *opt, int unset)
+{
+	struct strbuf *buf = opt->value;
+	const char *arg;
+
+	if (unset)
+		BUG("-F cannot be negated");
+
+	if (ctx->opt) {
+		arg = ctx->opt;
+		ctx->opt = NULL;
+	} else if (ctx->argc > 1) {
+		ctx->argc--;
+		arg = *++ctx->argv;
+	} else
+		return opterror(opt, "requires a value", 0);
+
+	if (buf->len)
+		strbuf_addch(buf, '\n');
+	if (ctx->prefix && !is_absolute_path(arg))
+		arg = prefix_filename(ctx->prefix, arg);
+	if (strbuf_read_file(buf, arg, 0) < 0)
+		return error(_("could not read file '%s'"), arg);
+	have_message = 1;
+
+	return 0;
+}
+
 static struct strategy *get_strategy(const char *name)
 {
 	int i;
@@ -228,6 +257,9 @@ static struct option builtin_merge_options[] = {
 	OPT_CALLBACK('m', "message", &merge_msg, N_("message"),
 		N_("merge commit message (for a non-fast-forward merge)"),
 		option_parse_message),
+	{ OPTION_LOWLEVEL_CALLBACK, 'F', "file", &merge_msg, N_("path"),
+		N_("read message from file"), PARSE_OPT_NONEG,
+		(parse_opt_cb *) option_read_message },
 	OPT__VERBOSITY(&verbosity),
 	OPT_BOOL(0, "abort", &abort_current_merge,
 		N_("abort the current in-progress merge")),