Allow the built-in exec path to be relative to the command invocation path

Allow the built-in exec path to be relative to the command invocation path

If GIT_EXEC_PATH (the macro that is defined in the Makefile) is relative,
it is interpreted relative to the command's invocation path, which usually
is $(bindir).

The Makefile rules were written with the assumption that $(gitexecdir) is
an absolute path. We introduce a separate variable that names the
(absolute) installation directory.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>