commit: --fixup option for use with rebase --autosquash

This option makes it convenient to construct commit messages for use
with 'rebase --autosquash'.  The resulting commit message will be
"fixup! ..." where "..." is the subject line of the specified commit
message.

Example usage:
  $ git commit --fixup HEAD~2

Signed-off-by: Pat Notz <patnotz@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-commit.txt

@@ -9,10 +9,10 @@ SYNOPSIS
 --------
 [verse]
 'git commit' [-a | --interactive] [-s] [-v] [-u<mode>] [--amend] [--dry-run]
-	   [(-c | -C) <commit>] [-F <file> | -m <msg>] [--reset-author]
+	   [(-c | -C | --fixup) <commit>] [-F <file> | -m <msg>]
-	   [--allow-empty] [--allow-empty-message] [--no-verify] [-e] [--author=<author>]
+	   [--reset-author] [--allow-empty] [--allow-empty-message] [--no-verify]
-	   [--date=<date>] [--cleanup=<mode>] [--status | --no-status] [--]
+	   [-e] [--author=<author>] [--date=<date>] [--cleanup=<mode>]
-	   [[-i | -o ]<file>...]
+	   [--status | --no-status] [--] [[-i | -o ]<file>...]
 
 DESCRIPTION
 -----------
@@ -70,6 +70,12 @@ OPTIONS
 	Like '-C', but with '-c' the editor is invoked, so that
 	the user can further edit the commit message.
 
+--fixup=<commit>::
+	Construct a commit message for use with `rebase --autosquash`.
+	The commit message will be the subject line from the specified
+	commit with a prefix of "fixup! ".  See linkgit:git-rebase[1]
+	for details.
+
 --reset-author::
 	When used with -C/-c/--amend options, declare that the
 	authorship of the resulting commit now belongs of the committer.

builtin/commit.c

@@ -69,6 +69,7 @@ static enum {
 static const char *logfile, *force_author;
 static const char *template_file;
 static char *edit_message, *use_message;
+static char *fixup_message;
 static char *author_name, *author_email, *author_date;
 static int all, edit_flag, also, interactive, only, amend, signoff;
 static int quiet, verbose, no_verify, allow_empty, dry_run, renew_authorship;
@@ -124,6 +125,7 @@ static struct option builtin_commit_options[] = {
 	OPT_CALLBACK('m', "message", &message, "MESSAGE", "specify commit message", opt_parse_m),
 	OPT_STRING('c', "reedit-message", &edit_message, "COMMIT", "reuse and edit message from specified commit"),
 	OPT_STRING('C', "reuse-message", &use_message, "COMMIT", "reuse message from specified commit"),
+	OPT_STRING(0, "fixup", &fixup_message, "COMMIT", "use autosquash formatted message to fixup specified commit"),
 	OPT_BOOLEAN(0, "reset-author", &renew_authorship, "the commit is authored by me now (used with -C-c/--amend)"),
 	OPT_BOOLEAN('s', "signoff", &signoff, "add Signed-off-by:"),
 	OPT_FILENAME('t', "template", &template_file, "use specified template file"),
@@ -586,6 +588,16 @@ static int prepare_to_commit(const char *index_file, const char *prefix,
 		strbuf_add(&sb, buffer + 2, strlen(buffer + 2));
 		hook_arg1 = "commit";
 		hook_arg2 = use_message;
+	} else if (fixup_message) {
+		struct pretty_print_context ctx = {0};
+		struct commit *commit;
+		commit = lookup_commit_reference_by_name(fixup_message);
+		if (!commit)
+			die("could not lookup commit %s", fixup_message);
+		ctx.output_encoding = get_commit_output_encoding();
+		format_commit_message(commit, "fixup! %s\n\n",
+				      &sb, &ctx);
+		hook_arg1 = "message";
 	} else if (!stat(git_path("MERGE_MSG"), &statbuf)) {
 		if (strbuf_read_file(&sb, git_path("MERGE_MSG"), 0) < 0)
 			die_errno("could not read MERGE_MSG");
@@ -863,7 +875,7 @@ static int parse_and_validate_options(int argc, const char *argv[],
 	if (force_author && renew_authorship)
 		die("Using both --reset-author and --author does not make sense");
 
-	if (logfile || message.len || use_message)
+	if (logfile || message.len || use_message || fixup_message)
 		use_editor = 0;
 	if (edit_flag)
 		use_editor = 1;
@@ -883,15 +895,17 @@ static int parse_and_validate_options(int argc, const char *argv[],
 		f++;
 	if (edit_message)
 		f++;
+	if (fixup_message)
+		f++;
 	if (logfile)
 		f++;
 	if (f > 1)
-		die("Only one of -c/-C/-F can be used.");
+		die("Only one of -c/-C/-F/--fixup can be used.");
 	if (message.len && f > 0)
-		die("Option -m cannot be combined with -c/-C/-F.");
+		die("Option -m cannot be combined with -c/-C/-F/--fixup.");
 	if (edit_message)
 		use_message = edit_message;
-	if (amend && !use_message)
+	if (amend && !use_message && !fixup_message)
 		use_message = "HEAD";
 	if (!use_message && renew_authorship)
 		die("--reset-author can be used only with -C, -c or --amend.");