This function replaces sequences of 'chmod +x' and 'git update-index
--chmod=+x' in the test suite, whose purpose is to help filesystems
that need core.filemode=false. Two places where only 'chmod +x' was used
we also use this new function.

The function calls 'git update-index --chmod' without checking
core.filemode (unlike some of the call sites did). We do this because the
call sites *expect* that the executable bit ends up in the index (ie. it
is not the purpose of the call sites to *test* whether git treats
'chmod +x' and 'update-index --chmod=+x' correctly). Therefore, on
filesystems with core.filemode=true the 'git update-index --chmod' is a
no-op.

The function uses --add with update-index to help one call site in
t6031-merge-recursive. It makes no difference for the other callers.
Signed-off-by: NJohannes Sixt <j6t@kdbg.org>