xdiff: change type of xdfile_t.changed from char to bool

xdiff: change type of xdfile_t.changed from char to bool

The only values possible for 'changed' is 1 and 0, which exactly maps
to a bool type. It might not look like this because action1 and action2
(which use to be dis1, and dis2) were also of type char and were
assigned numerical values within a few lines of 'changed' (what used to
be rchg).

Using DISCARD/KEEP/INVESTIGATE for action1[i]/action2[j], and true/false
for changed[k] makes it clear to future readers that these are
logically separate concepts.

Best-viewed-with: --color-words
Signed-off-by: Ezekiel Newren <ezekielnewren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>