Introduce the function strip_path_suffix()

The function strip_path_suffix() will try to strip a given suffix from
a given path.  The suffix must start at a directory boundary (i.e. "core"
is not a path suffix of "libexec/git-core", but "git-core" is).

Arbitrary runs of directory separators ("slashes") are assumed identical.

Example:

	prefix = strip_path_suffix("C:\\msysgit/\\libexec\\git-core",
			"libexec///git-core");

will set prefix to "C:\\msysgit".

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

cache.h

@@ -629,6 +629,7 @@ const char *make_nonrelative_path(const char *path);
 const char *make_relative_path(const char *abs, const char *base);
 int normalize_absolute_path(char *buf, const char *path);
 int longest_ancestor_length(const char *path, const char *prefix_list);
+char *strip_path_suffix(const char *path, const char *suffix);
 
 /* Read and unpack a sha1 file into memory, write memory to a sha1 file */
 extern int sha1_object_info(const unsigned char *, unsigned long *);

path.c

@@ -457,3 +457,35 @@ int longest_ancestor_length(const char *path, const char *prefix_list)
 
 	return max_len;
 }
+
+/* strip arbitrary amount of directory separators at end of path */
+static inline int chomp_trailing_dir_sep(const char *path, int len)
+{
+	while (len && is_dir_sep(path[len - 1]))
+		len--;
+	return len;
+}
+
+/* sets prefix if the suffix matches */
+char *strip_path_suffix(const char *path, const char *suffix)
+{
+	int path_len = strlen(path), suffix_len = strlen(suffix);
+
+	while (suffix_len) {
+		if (!path_len)
+			return NULL;
+
+		if (is_dir_sep(path[path_len - 1])) {
+			if (!is_dir_sep(suffix[suffix_len - 1]))
+				return NULL;
+			path_len = chomp_trailing_dir_sep(path, path_len);
+			suffix_len = chomp_trailing_dir_sep(suffix, suffix_len);
+		}
+		else if (path[--path_len] != suffix[--suffix_len])
+			return NULL;
+	}
+
+	if (path_len && !is_dir_sep(path[path_len - 1]))
+		return NULL;
+	return xstrndup(path, chomp_trailing_dir_sep(path, path_len));
+}

t/t0060-path-utils.sh

@@ -84,4 +84,8 @@ ancestor /foo/bar :://foo/.:: 4
 ancestor /foo/bar //foo/./::/bar 4
 ancestor /foo/bar ::/bar -1
 
+test_expect_success 'strip_path_suffix' '
+	test c:/msysgit = $(test-path-utils strip_path_suffix \
+		c:/msysgit/libexec//git-core libexec/git-core)
+'
 test_done

test-path-utils.c

@@ -22,5 +22,10 @@ int main(int argc, char **argv)
 		printf("%d\n", len);
 	}
 
+	if (argc == 4 && !strcmp(argv[1], "strip_path_suffix")) {
+		char *prefix = strip_path_suffix(argv[2], argv[3]);
+		printf("%s\n", prefix ? prefix : "(null)");
+	}
+
 	return 0;
 }