rebase: add an --am option

Currently, this option doesn't do anything except error out if any
options requiring the interactive-backend are also passed.  However,
when we make the default backend configurable later in this series, this
flag will provide a way to override the config setting.

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

Documentation/git-rebase.txt

@@ -258,6 +258,13 @@ See also INCOMPATIBLE OPTIONS below.
 	original branch. The index and working tree are also left
 	unchanged as a result.
 
+--am:
+	Use git-am internally to rebase.  This option may become a
+	no-op in the future once the interactive backend handles
+	everything the am one does.
++
+See also INCOMPATIBLE OPTIONS below.
+
 --empty={drop,keep,ask}::
 	How to handle commits that are not empty to start and are not
 	clean cherry-picks of any upstream commit, but which become
@@ -378,7 +385,7 @@ See also INCOMPATIBLE OPTIONS below.
 	Ensure at least <n> lines of surrounding context match before
 	and after each change.  When fewer lines of surrounding
 	context exist they all must match.  By default no context is
-	ever ignored.
+	ever ignored.  Implies --am.
 +
 See also INCOMPATIBLE OPTIONS below.
 
@@ -418,6 +425,7 @@ with `--keep-base` in order to drop those commits from your branch.
 --whitespace=<option>::
 	These flags are passed to the 'git apply' program
 	(see linkgit:git-apply[1]) that applies the patch.
+	Implies --am.
 +
 See also INCOMPATIBLE OPTIONS below.
 
@@ -561,6 +569,7 @@ INCOMPATIBLE OPTIONS
 
 The following options:
 
+ * --am
  * --committer-date-is-author-date
  * --ignore-date
  * --ignore-whitespace

builtin/rebase.c

@@ -1335,6 +1335,18 @@ done:
 	return res && is_linear_history(onto, head);
 }
 
+static int parse_opt_am(const struct option *opt, const char *arg, int unset)
+{
+	struct rebase_options *opts = opt->value;
+
+	BUG_ON_OPT_NEG(unset);
+	BUG_ON_OPT_ARG(arg);
+
+	opts->type = REBASE_AM;
+
+	return 0;
+}
+
 /* -i followed by -m is still -i */
 static int parse_opt_merge(const struct option *opt, const char *arg, int unset)
 {
@@ -1519,6 +1531,10 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 		OPT_CMDMODE(0, "show-current-patch", &action,
 			    N_("show the patch file being applied or merged"),
 			    ACTION_SHOW_CURRENT_PATCH),
+		{ OPTION_CALLBACK, 0, "am", &options, NULL,
+			N_("use apply-mail strategies to rebase"),
+			PARSE_OPT_NOARG | PARSE_OPT_NONEG,
+			parse_opt_am },
 		{ OPTION_CALLBACK, 'm', "merge", &options, NULL,
 			N_("use merging strategies to rebase"),
 			PARSE_OPT_NOARG | PARSE_OPT_NONEG,
@@ -1878,7 +1894,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 	if (isatty(2) && options.flags & REBASE_NO_QUIET)
 		strbuf_addstr(&options.git_format_patch_opt, " --progress");
 
-	if (options.git_am_opts.argc) {
+	if (options.git_am_opts.argc || options.type == REBASE_AM) {
 		/* all am options except -q are compatible only with --am */
 		for (i = options.git_am_opts.argc - 1; i >= 0; i--)
 			if (strcmp(options.git_am_opts.argv[i], "-q"))