Deduce exec_path also from calls to git with a relative path

There is already logic in the git wrapper to deduce the exec_path from
argv[0], when the git wrapper was called with an absolute path.  Extend
that logic to handle relative paths as well.

For example, when you call "../../hello/world/git", it will not turn
"../../hello/world" into an absolute path, and use that.

Initial implementation by Scott R Parish.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>