[PATCH v8 2/2] interpret_trailers: for three options parse add warning

["ZheNing Hu via GitGitGadget" <gitgitgadget@gmail.com>]

From: ZheNing Hu <adlternative@gmail.com>

When using `interpret-trailers`, if user accidentally fill in the
wrong option values (e.g. `--if-exists=addd` or `--where=begin` or
`--if-missing=do-nothing`), git will exit quietly, and the user may
not even know what happened.

So lets provides warnings when git parsing this three options:
"unknown value '%s' for key ('where'|'--if-exist'|'--if-missing')".
This will remind the user :"Oh, it was because of my spelling error."
(or using a synonym for error).

Signed-off-by: ZheNing Hu <adlternative@gmail.com>
---
 builtin/interpret-trailers.c | 25 ++++++++++++++++++++++---
 1 file changed, 22 insertions(+), 3 deletions(-)

diff --git a/builtin/interpret-trailers.c b/builtin/interpret-trailers.c
index 84748eafc01b..6d8e86a44d43 100644
--- a/builtin/interpret-trailers.c
+++ b/builtin/interpret-trailers.c
@@ -24,19 +24,38 @@ static enum trailer_if_missing if_missing;
 static int option_parse_where(const struct option *opt,
 			      const char *arg, int unset)
 {
-	return trailer_set_where(&where, arg);
+	int ret;
+
+	ret = trailer_set_where(&where, arg);
+	if (ret)
+		warning(_("unknown value '%s' for key 'where'"),
+			arg);
+	return ret;
+
 }
 
 static int option_parse_if_exists(const struct option *opt,
 				  const char *arg, int unset)
 {
-	return trailer_set_if_exists(&if_exists, arg);
+	int ret;
+
+	ret = trailer_set_if_exists(&if_exists, arg);
+	if (ret)
+		warning(_("unknown value '%s' for key 'if_exists'"),
+			arg);
+	return ret;
 }
 
 static int option_parse_if_missing(const struct option *opt,
 				   const char *arg, int unset)
 {
-	return trailer_set_if_missing(&if_missing, arg);
+	int ret;
+
+	ret = trailer_set_if_missing(&if_missing, arg);
+	if (ret)
+		warning(_("unknown value '%s' for key 'if_missing'"),
+			arg);
+	return ret;
 }
 
 static void new_trailers_clear(struct list_head *trailers)
-- 
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