set default timeout for git context

[crazy-max]

There are some cases where Git operations can be slow. For example checking dirty state can take more than 1 minute on WSL2 for fs operations on NTFS folders before the build even starts: microsoft/WSL#4197

$ cd /mnt/c/github.com/moby/moby
$ time git -c log.showSignature=false status --porcelain --ignored >/dev/null
real    1m5.620s
user    0m0.387s
sys     0m59.010s

Signed-off-by: CrazyMax crazy-max@users.noreply.github.com