Make running git under other debugger-like programs easy

Make running git under other debugger-like programs easy

This allows us to run git, when using the script from bin-wrappers, under
other programs.  A few examples for usage within testsuite scripts:

   debug git checkout master
   debug --debugger=nemiver git $ARGS
   debug -d "valgrind --tool-memcheck --track-origins=yes" git $ARGS

Or, if someone has bin-wrappers/ in their $PATH and is executing git
outside the testsuite:

   GIT_DEBUGGER="gdb --args" git $ARGS
   GIT_DEBUGGER=nemiver git $ARGS
   GIT_DEBUGGER="valgrind --tool=memcheck --track-origins=yes" git $ARGS

There is also a handy shortcut of GIT_DEBUGGER=1 meaning the same as
GIT_DEBUGGER="gdb --args"

Original-patch-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>