git-am: add am.threeWay config variable

Add the am.threeWay configuration variable to use the -3 or --3way
option of git am by default. When am.threeway is set and not desired
for a specific git am command, the --no-3way option can be used to
override it.

Signed-off-by: Remi Lespinet <remi.lespinet@ensimag.grenoble-inp.fr>
Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -769,6 +769,14 @@ am.keepcr::
 	by giving '--no-keep-cr' from the command line.
 	See linkgit:git-am[1], linkgit:git-mailsplit[1].
 
+am.threeWay::
+	By default, `git am` will fail if the patch does not apply cleanly. When
+	set to true, this setting tells `git am` to fall back on 3-way merge if
+	the patch records the identity of blobs it is supposed to apply to and
+	we have those blobs available locally (equivalent to giving the `--3way`
+	option from the command line). Defaults to `false`.
+	See linkgit:git-am[1].
+
 apply.ignoreWhitespace::
 	When set to 'change', tells 'git apply' to ignore changes in
 	whitespace, in the same way as the '--ignore-space-change'

Documentation/git-am.txt

@@ -10,7 +10,7 @@ SYNOPSIS
 --------
 [verse]
 'git am' [--signoff] [--keep] [--[no-]keep-cr] [--[no-]utf8]
-	 [--3way] [--interactive] [--committer-date-is-author-date]
+	 [--[no-]3way] [--interactive] [--committer-date-is-author-date]
 	 [--ignore-date] [--ignore-space-change | --ignore-whitespace]
 	 [--whitespace=<option>] [-C<n>] [-p<n>] [--directory=<dir>]
 	 [--exclude=<path>] [--include=<path>] [--reject] [-q | --quiet]
@@ -90,10 +90,13 @@ default.   You can use `--no-utf8` to override this.
 
 -3::
 --3way::
+--no-3way::
 	When the patch does not apply cleanly, fall back on
 	3-way merge if the patch records the identity of blobs
 	it is supposed to apply to and we have those blobs
-	available locally.
+	available locally. `--no-3way` can be used to override
+	am.threeWay configuration variable. For more information,
+	see am.threeWay in linkgit:git-config[1].
 
 --ignore-space-change::
 --ignore-whitespace::

builtin/am.c

@@ -152,6 +152,8 @@ static void am_state_init(struct am_state *state, const char *dir)
 
 	state->prec = 4;
 
+	git_config_get_bool("am.threeway", &state->threeway);
+
 	state->utf8 = 1;
 
 	git_config_get_bool("am.messageid", &state->message_id);

t/t4150-am.sh

@@ -551,6 +551,25 @@ test_expect_success 'am -3 -p0 can read --no-prefix patch' '
 	git diff --exit-code lorem
 '
 
+test_expect_success 'am with config am.threeWay falls back to 3-way merge' '
+	rm -fr .git/rebase-apply &&
+	git reset --hard &&
+	git checkout -b lorem4 base3way &&
+	test_config am.threeWay 1 &&
+	git am lorem-move.patch &&
+	test_path_is_missing .git/rebase-apply &&
+	git diff --exit-code lorem
+'
+
+test_expect_success 'am with config am.threeWay overridden by --no-3way' '
+	rm -fr .git/rebase-apply &&
+	git reset --hard &&
+	git checkout -b lorem5 base3way &&
+	test_config am.threeWay 1 &&
+	test_must_fail git am --no-3way lorem-move.patch &&
+	test_path_is_dir .git/rebase-apply
+'
+
 test_expect_success 'am can rename a file' '
 	grep "^rename from" rename.patch &&
 	rm -fr .git/rebase-apply &&