config: Fix --unset for continuation lines

find_beginning_of_line didn't take into account that the
previous line might have ended with \ in which case it shouldn't
stop but continue its search.

Signed-off-by: Frank Lichtenheld <frank@lichtenheld.de>
Acked-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.c

@@ -701,12 +701,17 @@ static ssize_t find_beginning_of_line(const char* contents, size_t size,
 	size_t equal_offset = size, bracket_offset = size;
 	ssize_t offset;
 
+contline:
 	for (offset = offset_-2; offset > 0
 			&& contents[offset] != '\n'; offset--)
 		switch (contents[offset]) {
 			case '=': equal_offset = offset; break;
 			case ']': bracket_offset = offset; break;
 		}
+	if (offset > 0 && contents[offset-1] == '\\') {
+		offset_ = offset;
+		goto contline;
+	}
 	if (bracket_offset < equal_offset) {
 		*found_bracket = 1;
 		offset = bracket_offset+1;

t/t1300-repo-config.sh

@@ -71,6 +71,25 @@ EOF
 
 test_expect_success 'non-match result' 'cmp .git/config expect'
 
+cat > .git/config <<\EOF
+[alpha]
+bar = foo
+[beta]
+baz = multiple \
+lines
+EOF
+
+test_expect_success 'unset with cont. lines' \
+	'git config --unset beta.baz'
+
+cat > expect <<\EOF
+[alpha]
+bar = foo
+[beta]
+EOF
+
+test_expect_success 'unset with cont. lines is correct' 'cmp .git/config expect'
+
 cat > .git/config << EOF
 [beta] ; silly comment # another comment
 noIndent= sillyValue ; 'nother silly comment