grepdiff: fails tests/grepdiff1 without pcre2 on musl

[hhoffstaette]

When built --without-pcre2 on musl the log says:

FAIL: tests/grepdiff1/run-test
==============================

+ cat
+ /var/cache/distfiles/patchutils/src/grepdiff '\+a' diff
+ exit 1
FAIL tests/grepdiff1/run-test (exit status: 1)

More info when running the test:

$./src/grepdiff '\+a' ./test-arena/grepdiff1/diff
./src/grepdiff:Repetition not preceded by valid expression

Expected output when built on glibc or --with-pcre2:

$./src/grepdiff '\+a' ./test-arena/grepdiff1/diff                       
file1

Apparently this is a difference between glibc and musl's regex implementations.

[hhoffstaette]

For some reason quoting the quoted expression seems to work:

$./src/grepdiff ''\+a'' ./test-arena/grepdiff1/diff
file1

[hhoffstaette]

Whoops! Duplicate of #61 fixed by #99