ignore duplicated slashes in make_relative_path()

The function takes two paths, an early part of abs is supposed to match
base; otherwise abs is not a path under base and the function returns the
full path of abs.  The caller can easily confuse the implementation by
giving duplicated and needless slashes in these path arguments.

Credit for test script, motivation and initial patch goes to Thomas Rast.
A follow-up fix (squashed) is by Hannes.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

path.c

diff --git a/path.c b/path.c
index 2ec950b..79aa104 100644
--- a/path.c
+++ b/path.c
@@ -394,17 +394,38 @@
 const char *make_relative_path(const char *abs, const char *base)
 {
 	static char buf[PATH_MAX + 1];
-	int baselen;
-	if (!base)
+	int i = 0, j = 0;
+
+	if (!base || !base[0])
 		return abs;
-	baselen = strlen(base);
-	if (prefixcmp(abs, base))
+	while (base[i]) {
+		if (is_dir_sep(base[i])) {
+			if (!is_dir_sep(abs[j]))
+				return abs;
+			while (is_dir_sep(base[i]))
+				i++;
+			while (is_dir_sep(abs[j]))
+				j++;
+			continue;
+		} else if (abs[j] != base[i]) {
+			return abs;
+		}
+		i++;
+		j++;
+	}
+	if (
+	    /* "/foo" is a prefix of "/foo" */
+	    abs[j] &&
+	    /* "/foo" is not a prefix of "/foobar" */
+	    !is_dir_sep(base[i-1]) && !is_dir_sep(abs[j])
+	   )
 		return abs;
-	if (abs[baselen] == '/')
-		baselen++;
-	else if (base[baselen - 1] != '/')
-		return abs;
-	strcpy(buf, abs + baselen);
+	while (is_dir_sep(abs[j]))
+		j++;
+	if (!abs[j])
+		strcpy(buf, ".");
+	else
+		strcpy(buf, abs + j);
 	return buf;
 }
 

t/t1501-worktree.sh

diff --git a/t/t1501-worktree.sh b/t/t1501-worktree.sh
index 74e6443..9df3012 100755
--- a/t/t1501-worktree.sh
+++ b/t/t1501-worktree.sh
@@ -189,4 +189,10 @@
 	)
 '
 
+test_expect_success 'make_relative_path handles double slashes in GIT_DIR' '
+	: > dummy_file
+	echo git --git-dir="$(pwd)//repo.git" --work-tree="$(pwd)" add dummy_file &&
+	git --git-dir="$(pwd)//repo.git" --work-tree="$(pwd)" add dummy_file
+'
+
 test_done