builtin rebase: stop if `git am` is in progress

This commit checks for the file `applying` used by `git am` in
`rebase-apply/` and if the file is present it means `git am` is in
progress so it errors out.

Signed-off-by: Pratik Karki <predatoramigo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/rebase.c

@@ -539,6 +539,11 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 
 	git_config(rebase_config, &options);
 
+	strbuf_reset(&buf);
+	strbuf_addf(&buf, "%s/applying", apply_dir());
+	if(file_exists(buf.buf))
+		die(_("It looks like 'git am' is in progress. Cannot rebase."));
+
 	if (is_directory(apply_dir())) {
 		options.type = REBASE_AM;
 		options.state_dir = apply_dir();