rebase -r: let label generate safer labels

The `label` todo command in interactive rebases creates temporary refs
in the `refs/rewritten/` namespace. These refs are stored as loose refs,
i.e. as files in `.git/refs/rewritten/`, therefore they have to conform
with file name limitations on the current filesystem in addition to the
accepted ref format.

This poses a problem in particular on NTFS/FAT, where e.g. the colon,
double-quote and pipe characters are disallowed as part of a file name.

Let's safeguard against this by replacing not only white-space
characters by dashes, but all non-alpha-numeric ones.

However, we exempt non-ASCII UTF-8 characters from that, as it should be
quite possible to reflect branch names such as `↯↯↯` in refs/file names.

Signed-off-by: Matthew Rogers <mattr94@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

sequencer.c

@@ -4473,8 +4473,26 @@ static const char *label_oid(struct object_id *oid, const char *label,
 	} else {
 		struct strbuf *buf = &state->buf;
 
+		/*
+		 * Sanitize labels by replacing non-alpha-numeric characters
+		 * (including white-space ones) by dashes, as they might be
+		 * illegal in file names (and hence in ref names).
+		 *
+		 * Note that we retain non-ASCII UTF-8 characters (identified
+		 * via the most significant bit). They should be all acceptable
+		 * in file names. We do not validate the UTF-8 here, that's not
+		 * the job of this function.
+		 */
 		for (; *label; label++)
-			strbuf_addch(buf, isspace(*label) ? '-' : *label);
+			if ((*label & 0x80) || isalnum(*label))
+				strbuf_addch(buf, *label);
+			/* avoid leading dash and double-dashes */
+			else if (buf->len && buf->buf[buf->len - 1] != '-')
+				strbuf_addch(buf, '-');
+		if (!buf->len) {
+			strbuf_addstr(buf, "rev-");
+			strbuf_add_unique_abbrev(buf, oid, default_abbrev);
+		}
 		label = buf->buf;
 
 		if ((buf->len == the_hash_algo->hexsz &&

t/t3430-rebase-merges.sh

@@ -468,4 +468,10 @@ test_expect_success '--rebase-merges with strategies' '
 	test_cmp expect G.t
 '
 
+test_expect_success '--rebase-merges with commit that can generate bad characters for filename' '
+	git checkout -b colon-in-label E &&
+	git merge -m "colon: this should work" G &&
+	git rebase --rebase-merges --force-rebase E
+'
+
 test_done