Move code interpreting path relative to exec-dir to new function system_path()

Expanding system paths relative to git_exec_path can be used for
creating an installation that can be moved to a different directory
without re-compiling.  We use this approach for template_dir and the
system wide gitconfig.  The Windows installer (msysgit) is an example
for such a setup.

This commit moves common code to a new function system_path().  System
paths that are to be interpreted relative to git_exec_path are passed to
system_path() and the return value is used instead of the original path.
system_path() prefixes a relative path with git_exec_path and leaves
absolute paths unmodified.  For example, we now write

    template_dir = system_path(DEFAULT_GIT_TEMPLATE_DIR);

[j6t: moved from path.c to exec_cmd.c]

Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

exec_cmd.h

@@ -6,6 +6,6 @@ extern const char* git_exec_path(void);
 extern void setup_path(const char *);
 extern int execv_git_cmd(const char **argv); /* NULL terminated */
 extern int execl_git_cmd(const char *cmd, ...);
-
+extern const char *system_path(const char *path);
 
 #endif /* GIT_EXEC_CMD_H */