scripts/setlocalversion on write-protected source tree

I just stumbled across another[0] issue when scripts/setlocalversion
operates on a write-protected source tree. Back then[0] the source tree
was on an read-only NFS share, so "test -w" was introduced before "git
update-index" was run.

This time, the source tree is on read/write NFS share, but the permissions
are world-readable and only a specific user (or root) can write.
Thus, "test -w ." returns "0" and then runs "git update-index",
producing the following message (on a dirty tree):

  fatal: Unable to create '/usr/local/src/linux-git/.git/index.lock': Permission denied

While it says "fatal", compilation continues just fine.

However, I don't think a kernel compilation should alter the source
tree (or the .git directory) in any way and I don't see how removing
"git update-index" could do any harm. The Mercurial and SVN routines in
scripts/setlocalversion don't have any tree-modifying commands, AFAICS.
So, maybe the patch below would be acceptable.

[0] https://patchwork.kernel.org/patch/29718/

Signed-off-by: Christian Kujau <lists@nerdbynature.de>
Cc: Nico Schottelius <nico-linuxsetlocalversion@schottelius.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>