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>

git.c

@@ -409,13 +409,14 @@ int main(int argc, const char **argv)
 	/*
 	 * Take the basename of argv[0] as the command
 	 * name, and the dirname as the default exec_path
-	 * if it's an absolute path and we don't have
-	 * anything better.
+	 * if we don't have anything better.
 	 */
 	if (slash) {
 		*slash++ = 0;
 		if (*cmd == '/')
 			exec_path = cmd;
+		else
+			exec_path = xstrdup(make_absolute_path(cmd));
 		cmd = slash;
 	}