vcs-svn: avoid self-assignment in dummy initialization of pre_off

vcs-svn: avoid self-assignment in dummy initialization of pre_off

Without this change, clang complains:

 vcs-svn/svndiff.c:298:3: warning: Assigned value is garbage or undefined
                 off_t pre_off = pre_off; /* stupid GCC... */
                 ^               ~~~~~~~

This code uses an old and common idiom for suppressing an
"uninitialized variable" warning, and clang is wrong to warn about it.
The idiom tells the compiler to leave the variable uninitialized,
which saves a few bytes of code size, and, more importantly, allows
valgrind to check at runtime that the variable is properly initialized
by the time it is used.

But MSVC and clang do not know that idiom, so let's avoid it in
vcs-svn/ code.

Initialize pre_off to -1, a recognizably meaningless value, to allow
future code changes that cause pre_off to be used before it is
initialized to be caught early.

Signed-off-by: David Barr <davidbarr@google.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>