check-ref-format --print: Normalize refnames that start with slashes

When asked if "refs///heads/master" is valid, check-ref-format says "Yes,
it is well formed", and when asked to print canonical form, it shows
"refs/heads/master". This is so that it can be tucked after "$GIT_DIR/"
to form a valid pathname for a loose ref, and we normalize a pathname like
"$GIT_DIR/refs///heads/master" to de-dup the slashes in it.

Similarly, when asked if "/refs/heads/master" is valid, check-ref-format
says "Yes, it is Ok", but the leading slash is not removed when printing,
leading to "$GIT_DIR//refs/heads/master".

Fix it to make sure such leading slashes are removed.  Add tests that such
refnames are accepted and normalized correctly.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/check-ref-format.c

@@ -12,8 +12,8 @@ static const char builtin_check_ref_format_usage[] =
 "   or: git check-ref-format --branch <branchname-shorthand>";
 
 /*
- * Replace each run of adjacent slashes in src with a single slash,
- * and write the result to dst.
+ * Remove leading slashes and replace each run of adjacent slashes in
+ * src with a single slash, and write the result to dst.
  *
  * This function is similar to normalize_path_copy(), but stripped down
  * to meet check_ref_format's simpler needs.
@@ -21,7 +21,7 @@ static const char builtin_check_ref_format_usage[] =
 static void collapse_slashes(char *dst, const char *src)
 {
 	char ch;
-	char prev = '\0';
+	char prev = '/';
 
 	while ((ch = *src++) != '\0') {
 		if (prev == '/' && ch == prev)

t/t1402-check-ref-format.sh

@@ -18,6 +18,9 @@ invalid_ref 'foo'
 valid_ref 'foo/bar/baz'
 valid_ref 'refs///heads/foo'
 invalid_ref 'heads/foo/'
+valid_ref '/heads/foo'
+valid_ref '///heads/foo'
+invalid_ref '/foo'
 invalid_ref './foo'
 invalid_ref '.refs/foo'
 invalid_ref 'heads/foo..bar'
@@ -70,7 +73,10 @@ invalid_ref_normalized() {
 
 valid_ref_normalized 'heads/foo' 'heads/foo'
 valid_ref_normalized 'refs///heads/foo' 'refs/heads/foo'
+valid_ref_normalized '/heads/foo' 'heads/foo'
+valid_ref_normalized '///heads/foo' 'heads/foo'
 invalid_ref_normalized 'foo'
+invalid_ref_normalized '/foo'
 invalid_ref_normalized 'heads/foo/../bar'
 invalid_ref_normalized 'heads/./foo'
 invalid_ref_normalized 'heads\foo'