Fix regression when running tig with no arguments and custom encoding

Fix regression when running tig with no arguments and custom encoding

Since c7d67ab running "tig" with no options has failed with the error
"tig: No revisions match the given arguments." This was due to a change
in how the arguments for the back-end git call was being constructed.
This change caused the blank field left in place of "%(encoding_arg)"
when it is empty to not overwrite "buf" which then caused the value in
"buf" to be copied into dst_argv twice. The resulting git command failed
if there was no available revision named "log" as shown in the trace.

From the TIG_TRACE log:
git log log --no-color --pretty=raw --parents --parents --
fatal: bad revision 'log'

This fix works by properly and fully initializing "buf" before each use.

Fixes #167

Signed-off-by: Drew Northup <n1xim.email@gmail.com>
Minor edits of commit message and change patch to use sizeof(buf).

Signed-off-by: Jonas Fonseca <fonseca@diku.dk>