commit-tree: utilize parse-options api

Rather than parse options manually, which is both difficult to
read and error prone, parse options supplied to commit-tree
using the parse-options api.

It was discovered that the --no-gpg-sign option was documented
but not implemented in commit 70ddbd7767 (commit-tree: add missing
--gpg-sign flag, 2019-01-19), and the existing implementation
would attempt to translate the option as a tree oid. It was also
suggested earlier in commit 55ca3f99ae (commit-tree: add and document
--no-gpg-sign, 2013-12-13) that commit-tree should be migrated to
utilize the parse-options api, which could help prevent mistakes
like this in the future. Hence this change.

Also update the documentation to better describe that mixing
`-m` and `-F` options will correctly compose commit log messages in the
order in which the options are given.

In the process, mark various strings for translation.

Signed-off-by: Brandon Richardson <brandon1024.br@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

parse-options.h

@@ -202,6 +202,17 @@ const char *optname(const struct option *opt, int flags);
 		BUG("option callback does not expect an argument"); \
 } while (0)
 
+/*
+ * Similar to the assertions above, but checks that "arg" is always non-NULL.
+ * This assertion also implies BUG_ON_OPT_NEG(), letting you declare both
+ * assertions in a single line.
+ */
+#define BUG_ON_OPT_NEG_NOARG(unset, arg) do { \
+	BUG_ON_OPT_NEG(unset); \
+	if(!(arg)) \
+		BUG("option callback expects an argument"); \
+} while(0)
+
 /*----- incremental advanced APIs -----*/
 
 enum {