Avoid multiple PREFIX definitions

Avoid multiple PREFIX definitions

The short and sweet PREFIX can be confused when used in many places.

Rename both usages to better describe their purpose. EXEC_CMD_PREFIX is
used in full to disambiguate it from the nearby GIT_EXEC_PATH.

The PREFIX in sideband.c, while nominally independant of the exec_cmd
PREFIX, does reside within libgit[1], so the definitions would clash
when taken together with a PREFIX given on the command line for use by
exec_cmd.c.

Noticed when compiling Git for Windows using MSVC/Visual Studio [1] which
reports the conflict beteeen the command line definition and the
definition in sideband.c within the libgit project.

[1] the libgit functions are brought into a single sub-project
within the Visual Studio construction script provided in contrib,
and hence uses a single command for both exec_cmd.c and sideband.c.

Signed-off-by: Philip Oakley <philipoakley@iee.org>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>