config: do not use C function names as struct members

According to C99, section 7.1.4:

  Any function declared in a header may be additionally
  implemented as a function-like macro defined in the
  header.

Therefore calling our struct member function pointer "fgetc"
may run afoul of unwanted macro expansion when we call:

  char c = cf->fgetc(cf);

This turned out to be a problem on uclibc, which defines
fgetc as a macro and causes compilation failure.

The standard suggests fixing this in a few ways:

  1. Using extra parentheses to inhibit the function-like
     macro expansion. E.g., "(cf->fgetc)(cf)". This is
     undesirable as it's ugly, and each call site needs to
     remember to use it (and on systems without the macro,
     forgetting will compile just fine).

  2. Using #undef (because a conforming implementation must
     also be providing fgetc as a function). This is
     undesirable because presumably the implementation was
     using the macro for a performance benefit, and we are
     dropping that optimization.

Instead, we can simply use non-colliding names.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>