meson: fix overwritten `git` variable

meson: fix overwritten `git` variable

We're assigning the `git` variable in three places:

  - In "meson.build" to store the external Git executable.

  - In "meson.build" to store the compiled Git executable.

  - In "Documentation/meson.build" to store the external Git executable,
    a second time.

The last case is only needed because we overwrite the original variable
with the built version. Rename the variable used for the built Git
executable so that we don't have to resolve the external Git executable
multiple times.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>