diff: update conflict handling for whitespace to issue a warning

Modify the conflict resolution between tab-in-indent and
indent-with-non-tab to issue a warning instead of terminating
the operation with `die()`. Update the `git diff --check` test to
capture and verify the warning message output.

Suggested-by: Phillip Wood <[email protected]>
Signed-off-by: Usman Akinyemi <[email protected]>

Author: Unique-Usman

t/t4015-diff-whitespace.sh

@@ -808,7 +808,8 @@ test_expect_success 'ditto, but tabwidth=1 (must be irrelevant)' '
 test_expect_success 'check tab-in-indent and indent-with-non-tab conflict' '
 	git config core.whitespace "tab-in-indent,indent-with-non-tab" &&
 	echo "foo ();" >x &&
-	test_must_fail git diff --check
+	git diff --check 2>error &&
+	test_grep "warning: cannot enforce both tab-in-indent and indent-with-non-tab, removing tab-in-indent" error
 '
 
 test_expect_success 'check tab-in-indent excluded from wildcard whitespace attribute' '

ws.c

@@ -6,6 +6,7 @@
 #include "git-compat-util.h"
 #include "attr.h"
 #include "strbuf.h"
+#include "gettext.h"
 #include "ws.h"
 
 unsigned whitespace_rule_cfg = WS_DEFAULT_RULE;
@@ -70,8 +71,10 @@ unsigned parse_whitespace_rule(const char *string)
 		string = ep;
 	}
 
-	if (rule & WS_TAB_IN_INDENT && rule & WS_INDENT_WITH_NON_TAB)
-		die("cannot enforce both tab-in-indent and indent-with-non-tab");
+	if (rule & WS_TAB_IN_INDENT && rule & WS_INDENT_WITH_NON_TAB) {
+		warning(_("cannot enforce both tab-in-indent and indent-with-non-tab, removing tab-in-indent"));
+		rule &= ~WS_TAB_IN_INDENT;
+	}
 	return rule;
 }