upload-pack: Initialize the exec-path.

upload-pack: Initialize the exec-path.

Since git-upload-pack has to spawn git-pack-objects, it has to make sure
that the latter can be found in the PATH. Without this patch an attempt
to clone or pull via ssh from a server fails if the git tools are not in
the standard PATH on the server even though git clone or git pull were
invoked with --upload-pack=/path/to/git-upload-pack.

Signed-off-by: Johannes Sixt <redacted>
Signed-off-by: Junio C Hamano <redacted>