add -p: avoid ambiguous signed/unsigned comparison

In the interactive `add` operation, users can choose to jump to specific
hunks, and Git will present the hunk list in that case. To avoid showing
too many lines at once, only a maximum of 21 hunks are shown, skipping
the "mode change" pseudo hunk.

The comparison performed to skip the "mode change" pseudo hunk (if any)
compares a signed integer `i` to the unsigned value `mode_change` (which
can be 0 or 1 because it is a 1-bit type).

According to section 6.3.1.8 of the C99 standard (see e.g.
https://www.open-std.org/jtc1/sc22/WG14/www/docs/n1256.pdf), what should
happen is an automatic conversion of the "lesser" type to the "greater"
type, but since the types differ in signedness, it is ill-defined what
is the correct "usual arithmetic conversion".

Which means that Visual C's behavior can (and does) differ from GCC's:
When compiling Git using the latter, `add -p`'s `goto` command shows no
hunks by default because it casts a negative start offset to a pretty
large unsigned value, breaking the "goto hunk" test case in
`t3701-add-interactive.sh`.

Let's avoid that by converting the unsigned bit explicitly to a signed
integer.

Note: This is a long-standing bug in the Visual C build of Git, but it
has never been caught because t3701 is skipped when `NO_PERL` is set,
which is the case in the `vs-test` jobs of Git's CI runs.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

add-patch.c

@@ -1547,7 +1547,7 @@ soft_increment:
 			strbuf_remove(&s->answer, 0, 1);
 			strbuf_trim(&s->answer);
 			i = hunk_index - DISPLAY_HUNKS_LINES / 2;
-			if (i < file_diff->mode_change)
+			if (i < (int)file_diff->mode_change)
 				i = file_diff->mode_change;
 			while (s->answer.len == 0) {
 				i = display_hunks(s, file_diff, i);