-Wuninitialized: remove some 'init-self' workarounds

-Wuninitialized: remove some 'init-self' workarounds

The 'self-initialised' variables construct (ie <type> var = var;) has
been used to silence gcc '-W[maybe-]uninitialized' warnings. This has,
unfortunately, caused MSVC to issue 'uninitialized variable' warnings.
Also, using clang static analysis causes complaints about an 'Assigned
value is garbage or undefined'.

There are six such constructs in the current codebase. Only one of the
six causes gcc to issue a '-Wmaybe-uninitialized' warning (which will
be addressed elsewhere). The remaining five 'init-self' gcc workarounds
are noted below, along with the commit which introduced them:

  1. builtin/rev-list.c: 'reaches' and 'all', see commit 457f08a030
     ("git-rev-list: add --bisect-vars option.", 2007-03-21).

  2. merge-recursive.c:2064 'mrtree', see commit f120ae2a8e ("merge-
     recursive.c: mrtree in merge() is not used before set", 2007-10-29).

  3. fast-import.c:3023 'oe', see commit 85c62395b1 ("fast-import: let
     importers retrieve blobs", 2010-11-28).

  4. fast-import.c:3006 'oe', see commit 28c7b1f7b7 ("fast-import: add a
     get-mark command", 2015-07-01).

Remove the 'self-initialised' variable constructs noted above.

Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>