commit: --squash 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
"squash! ..." where "..." is the subject line of the specified commit
message.  This option can be used with other commit message options
such as -m, -c, -C and -F.

If an editor is invoked (as with -c or -eF or no message options) the
commit message is seeded with the correctly formatted subject line.

Example usage:
  $ git commit --squash HEAD~2
  $ git commit --squash HEAD~2 -m "clever comment"
  $ git commit --squash HEAD~2 -F msgfile
  $ git commit --squash HEAD~2 -C deadbeef
Signed-off-by: NPat Notz <patnotz@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-commit.txt

@@ -9,7 +9,7 @@ SYNOPSIS
 --------
 [verse]
 'git commit' [-a | --interactive] [-s] [-v] [-u<mode>] [--amend] [--dry-run]
-	   [(-c | -C | --fixup) <commit>] [-F <file> | -m <msg>]
+	   [(-c | -C | --fixup | --squash) <commit>] [-F <file> | -m <msg>]
 	   [--reset-author] [--allow-empty] [--allow-empty-message] [--no-verify]
 	   [-e] [--author=<author>] [--date=<date>] [--cleanup=<mode>]
 	   [--status | --no-status] [--] [[-i | -o ]<file>...]
@@ -76,6 +76,13 @@ OPTIONS
 	commit with a prefix of "fixup! ".  See linkgit:git-rebase[1]
 	for details.
 
+--squash=<commit>::
+	Construct a commit message for use with `rebase --autosquash`.
+	The commit message subject line is taken from the specified
+	commit with a prefix of "squash! ".  Can be used with additional
+	commit message options (`-m`/`-c`/`-C`/`-F`). 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

@@ -69,7 +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 *fixup_message, *squash_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;
@@ -126,6 +126,7 @@ static struct option builtin_commit_options[] = {
 	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_STRING(0, "squash", &squash_message, "COMMIT", "use autosquash formatted message to squash 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"),
@@ -567,6 +568,25 @@ static int prepare_to_commit(const char *index_file, const char *prefix,
 	if (!no_verify && run_hook(index_file, "pre-commit", NULL))
 		return 0;
 
+	if (squash_message) {
+		/*
+		 * Insert the proper subject line before other commit
+		 * message options add their content.
+		 */
+		if (use_message && !strcmp(use_message, squash_message))
+			strbuf_addstr(&sb, "squash! ");
+		else {
+			struct pretty_print_context ctx = {0};
+			struct commit *c;
+			c = lookup_commit_reference_by_name(squash_message);
+			if (!c)
+				die("could not lookup commit %s", squash_message);
+			ctx.output_encoding = get_commit_output_encoding();
+			format_commit_message(c, "squash! %s\n\n", &sb,
+					      &ctx);
+		}
+	}
+
 	if (message.len) {
 		strbuf_addbuf(&sb, &message);
 		hook_arg1 = "message";
@@ -619,6 +639,16 @@ static int prepare_to_commit(const char *index_file, const char *prefix,
 	else if (in_merge)
 		hook_arg1 = "merge";
 
+	if (squash_message) {
+		/*
+		 * If squash_commit was used for the commit subject,
+		 * then we're possibly hijacking other commit log options.
+		 * Reset the hook args to tell the real story.
+		 */
+		hook_arg1 = "message";
+		hook_arg2 = "";
+	}
+
 	fp = fopen(git_path(commit_editmsg), "w");
 	if (fp == NULL)
 		die_errno("could not open '%s'", git_path(commit_editmsg));
@@ -890,7 +920,8 @@ static int parse_and_validate_options(int argc, const char *argv[],
 		die("You have nothing to amend.");
 	if (amend && in_merge)
 		die("You are in the middle of a merge -- cannot amend.");
-
+	if (fixup_message && squash_message)
+		die("Options --squash and --fixup cannot be used together");
 	if (use_message)
 		f++;
 	if (edit_message)