Makefile: insert SANE_TOOL_PATH to PATH before /bin or /usr/bin

Makefile: insert SANE_TOOL_PATH to PATH before /bin or /usr/bin

In an earlier patch, we introduced SANE_TOOL_PATH that is prepended to
user's PATH.  This had an unintended consequence of overriding user's
private binary directory that typically comes earlier in the PATH to holds
even saner commands than whatever comes with the system.

For example, a user may have ~/bin that is early in the path and contains
a shell script "vi" that launches system's /bin/vi with specific options.
Prepending SANE_TOOL_PATH to the PATH that happens to have "vi" in it
defeats such customization.

This fixes the issue by inserting SANE_TOOL_PATH just before /bin or
/usr/bin appears on the PATH.

Signed-off-by: Junio C Hamano <gitster@pobox.com>