Make prefix_path() return char* without const

prefix_path() allocates new buffer. There's no reason for it to keep
the buffer for itself and waste memory.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -428,7 +428,7 @@ extern const char **get_pathspec(const char *prefix, const char **pathspec);
 extern void setup_work_tree(void);
 extern const char *setup_git_directory_gently(int *);
 extern const char *setup_git_directory(void);
-extern const char *prefix_path(const char *prefix, int len, const char *path);
+extern char *prefix_path(const char *prefix, int len, const char *path);
 extern const char *prefix_filename(const char *prefix, int len, const char *path);
 extern int check_filename(const char *prefix, const char *name);
 extern void verify_filename(const char *prefix, const char *name);

setup.c

@@ -4,7 +4,7 @@
 static int inside_git_dir = -1;
 static int inside_work_tree = -1;
 
-const char *prefix_path(const char *prefix, int len, const char *path)
+char *prefix_path(const char *prefix, int len, const char *path)
 {
 	const char *orig = path;
 	char *sanitized = xmalloc(len + strlen(path) + 1);