merge.conflictstyle: choose between "merge" and "diff3 -m" styles

This teaches "git merge-file" to honor merge.conflictstyle configuration
variable, whose value can be "merge" (default) or "diff3".

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -889,6 +889,14 @@ man.<tool>.path::
 	Override the path for the given tool that may be used to
 	display help in the 'man' format. See linkgit:git-help[1].
 
+merge.conflictstyle::
+	Specify the style in which conflicted hunks are written out to
+	working tree files upon merge.  The default is "merge", which
+	shows `<<<<<<<` conflict marker, change made by one side,
+	`=======` marker, change made by the other side, and then
+	`>>>>>>>` marker.  An alternate style, "diff3", adds `|||||||`
+	marker and the original text before `=======` marker.
+
 mergetool.<tool>.path::
 	Override the path for the given tool.  This is useful in case
 	your tool is not in the PATH.

builtin-merge-file.c

@@ -15,6 +15,15 @@ int cmd_merge_file(int argc, const char **argv, const char *prefix)
 	int ret = 0, i = 0, to_stdout = 0;
 	int merge_level = XDL_MERGE_ZEALOUS_ALNUM;
 	int merge_style = 0;
+	int nongit;
+
+	prefix = setup_git_directory_gently(&nongit);
+	if (!nongit) {
+		/* Read the configuration file */
+		git_config(git_xmerge_config, NULL);
+		if (0 <= git_xmerge_style)
+			merge_style = git_xmerge_style;
+	}
 
 	while (argc > 4) {
 		if (!strcmp(argv[1], "-L") && i < 3) {

t/t6023-merge-file.sh

@@ -192,10 +192,17 @@ non timebo mala, quoniam TU mecum es:
 virga tua et baculus tuus ipsa me consolata sunt.
 EOF
 
-test_expect_success '"diff3 -m" style output' '
+test_expect_success '"diff3 -m" style output (1)' '
 	test_must_fail git merge-file -p --diff3 \
 		new8.txt new5.txt new9.txt >actual &&
 	test_cmp expect actual
 '
 
+test_expect_success '"diff3 -m" style output (2)' '
+	git config merge.conflictstyle diff3 &&
+	test_must_fail git merge-file -p \
+		new8.txt new5.txt new9.txt >actual &&
+	test_cmp expect actual
+'
+
 test_done

xdiff-interface.c

@@ -237,3 +237,23 @@ void xdiff_set_find_func(xdemitconf_t *xecfg, const char *value)
 		value = ep + 1;
 	}
 }
+
+int git_xmerge_style = -1;
+
+int git_xmerge_config(const char *var, const char *value, void *cb)
+{
+	if (!strcasecmp(var, "merge.conflictstyle")) {
+		if (!value)
+			die("'%s' is not a boolean", var);
+		if (!strcmp(value, "diff3"))
+			git_xmerge_style = XDL_MERGE_DIFF3;
+		else if (!strcmp(value, "merge"))
+			git_xmerge_style = 0;
+		else
+			die("unknown style '%s' given for '%s'",
+			    value, var);
+		return 0;
+	}
+	return git_default_config(var, value, cb);
+}
+

xdiff-interface.h

@@ -22,5 +22,7 @@ int read_mmfile(mmfile_t *ptr, const char *filename);
 int buffer_is_binary(const char *ptr, unsigned long size);
 
 extern void xdiff_set_find_func(xdemitconf_t *xecfg, const char *line);
+extern int git_xmerge_config(const char *var, const char *value, void *cb);
+extern int git_xmerge_style;
 
 #endif