help_unknown_cmd: do not propose an "unknown" cmd

When executing an external shell script like `git foo` with a bad
shebang, e.g. "#!/usr/bin/not/existing", execvp returns 127 (ENOENT).

Since help_unknown_cmd proposes the use of all external commands similar
to the name of the "unknown" command, it suggests the just failed command
again. Stop it and give some advice to the user.
Helped-by: NJeff King <peff@peff.net>
Signed-off-by: NMichael Schubert <mschub@elegosoft.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

help.c

@@ -302,6 +302,10 @@ static void add_cmd_list(struct cmdnames *cmds, struct cmdnames *old)
 #define SIMILARITY_FLOOR 7
 #define SIMILAR_ENOUGH(x) ((x) < SIMILARITY_FLOOR)
 
+static const char bad_interpreter_advice[] =
+	N_("'%s' appears to be a git command, but we were not\n"
+	"able to execute it. Maybe git-%s is broken?");
+
 const char *help_unknown_cmd(const char *cmd)
 {
 	int i, n, best_similarity = 0;
@@ -326,6 +330,14 @@ const char *help_unknown_cmd(const char *cmd)
 		int cmp = 0; /* avoid compiler stupidity */
 		const char *candidate = main_cmds.names[i]->name;
 
+		/*
+		 * An exact match means we have the command, but
+		 * for some reason exec'ing it gave us ENOENT; probably
+		 * it's a bad interpreter in the #! line.
+		 */
+		if (!strcmp(candidate, cmd))
+			die(_(bad_interpreter_advice), cmd, cmd);
+
 		/* Does the candidate appear in common_cmds list? */
 		while (n < ARRAY_SIZE(common_cmds) &&
 		       (cmp = strcmp(common_cmds[n].name, candidate)) < 0)