t4061: use POSIX compliant regex(7)

BRE interprets `+` literally, and
`\+` is undefined for POSIX BRE, from:
https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html#tag_09_03_02

> The interpretation of an ordinary character preceded
> by an unescaped <backslash> ( '\\' ) is undefined, except for:
> - The characters ')', '(', '{', and '}'
> - The digits 1 to 9 inclusive
> - A character inside a bracket expression

This test is failing with busybox sed, the default sed of Alpine Linux

We have 2 options here:

- Using literal `+` because BRE will interpret it as-is, or
- Using character class `[+]` to defend against a sed that expects ERE

ERE-expected sed is theoretical at this point,
but we haven't found it, yet.
And, we may run into other problems with that sed.
Let's go with first option and fix it later if that sed could be found.

Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4061-diff-indent.sh

@@ -17,7 +17,7 @@ compare_diff () {
 # Compare blame output using the expectation for a diff as reference.
 # Only look for the lines coming from non-boundary commits.
 compare_blame () {
-	sed -n -e "1,4d" -e "s/^\+//p" <"$1" >.tmp-1
+	sed -n -e "1,4d" -e "s/^+//p" <"$1" >.tmp-1
 	sed -ne "s/^[^^][^)]*) *//p" <"$2" >.tmp-2
 	test_cmp .tmp-1 .tmp-2 && rm -f .tmp-1 .tmp-2
 }