am: support --show-current-patch=diff to retrieve .git/rebase-apply/patch

When "git am --show-current-patch" was added in commit 984913a210 ("am:
add --show-current-patch", 2018-02-12), "git am" started recommending it
as a replacement for .git/rebase-merge/patch.  Unfortunately the suggestion
is somewhat misguided; for example, the output of "git am --show-current-patch"
cannot be passed to "git apply" if it is encoded as quoted-printable
or base64.  Add a new mode to "git am --show-current-patch" in order to
straighten the suggestion.

Reported-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-am.txt

@@ -16,7 +16,7 @@ SYNOPSIS
 	 [--exclude=<path>] [--include=<path>] [--reject] [-q | --quiet]
 	 [--[no-]scissors] [-S[<keyid>]] [--patch-format=<format>]
 	 [(<mbox> | <Maildir>)...]
-'git am' (--continue | --skip | --abort | --quit | --show-current-patch[=raw])
+'git am' (--continue | --skip | --abort | --quit | --show-current-patch[=(diff|raw)])
 
 DESCRIPTION
 -----------
@@ -176,10 +176,11 @@ default.   You can use `--no-utf8` to override this.
 	Abort the patching operation but keep HEAD and the index
 	untouched.
 
---show-current-patch[=raw]::
-	Show the raw contents of the e-mail message at which `git am`
-	has stopped due to conflicts.  The argument must be omitted or
-	`raw`.
+--show-current-patch[=(diff|raw)]::
+	Show the message at which `git am` has stopped due to
+	conflicts.  If `raw` is specified, show the raw contents of
+	the e-mail message; if `diff`, show the diff portion only.
+	Defaults to `raw`.
 
 DISCUSSION
 ----------

builtin/am.c

@@ -83,6 +83,7 @@ enum signoff_type {
 
 enum show_patch_type {
 	SHOW_PATCH_RAW = 0,
+	SHOW_PATCH_DIFF = 1,
 };
 
 struct am_state {
@@ -1767,7 +1768,7 @@ static void am_run(struct am_state *state, int resume)
 				linelen(state->msg), state->msg);
 
 			if (advice_amworkdir)
-				advise(_("Use 'git am --show-current-patch' to see the failed patch"));
+				advise(_("Use 'git am --show-current-patch=diff' to see the failed patch"));
 
 			die_user_resolve(state);
 		}
@@ -2086,6 +2087,9 @@ static int show_patch(struct am_state *state, enum show_patch_type sub_mode)
 	case SHOW_PATCH_RAW:
 		patch_path = am_path(state, msgnum(state));
 		break;
+	case SHOW_PATCH_DIFF:
+		patch_path = am_path(state, "patch");
+		break;
 	default:
 		BUG("invalid mode for --show-current-patch");
 	}
@@ -2154,6 +2158,7 @@ static int parse_opt_show_current_patch(const struct option *opt, const char *ar
 	 * when you add new options
 	 */
 	const char *valid_modes[] = {
+		[SHOW_PATCH_DIFF] = "diff",
 		[SHOW_PATCH_RAW] = "raw"
 	};
 	int new_value = SHOW_PATCH_RAW;
@@ -2279,7 +2284,7 @@ int cmd_am(int argc, const char **argv, const char *prefix)
 			N_("abort the patching operation but keep HEAD where it is."),
 			RESUME_QUIT),
 		{ OPTION_CALLBACK, 0, "show-current-patch", &resume.mode,
-		  "raw",
+		  "(diff|raw)",
 		  N_("show the patch being applied"),
 		  PARSE_OPT_CMDMODE | PARSE_OPT_OPTARG | PARSE_OPT_NONEG | PARSE_OPT_LITERAL_ARGHELP,
 		  parse_opt_show_current_patch, RESUME_SHOW_PATCH },

contrib/completion/git-completion.bash

@@ -1180,7 +1180,7 @@ __git_count_arguments ()
 
 __git_whitespacelist="nowarn warn error error-all fix"
 __git_patchformat="mbox stgit stgit-series hg mboxrd"
-__git_showcurrentpatch="raw"
+__git_showcurrentpatch="diff raw"
 __git_am_inprogress_options="--skip --continue --resolved --abort --quit --show-current-patch"
 
 _git_am ()

t/t4150-am.sh

@@ -671,11 +671,21 @@ test_expect_success 'am --show-current-patch=raw' '
 	test_cmp .git/rebase-apply/0001 actual.patch
 '
 
+test_expect_success 'am --show-current-patch=diff' '
+	git am --show-current-patch=diff >actual.patch &&
+	test_cmp .git/rebase-apply/patch actual.patch
+'
+
 test_expect_success 'am accepts repeated --show-current-patch' '
 	git am --show-current-patch --show-current-patch=raw >actual.patch &&
 	test_cmp .git/rebase-apply/0001 actual.patch
 '
 
+test_expect_success 'am detects incompatible --show-current-patch' '
+	test_must_fail git am --show-current-patch=raw --show-current-patch=diff &&
+	test_must_fail git am --show-current-patch --show-current-patch=diff
+'
+
 test_expect_success 'am --skip works' '
 	echo goodbye >expected &&
 	git am --skip &&