Add option to prepend line numbers to Line

[darkdread]

This change allows user to set an option to prepend line numbers to line output in diffs. I use it for prompting.
An example output:

diff --git a/builtin-http-fetch.c b/http-fetch.c
similarity index 95%
rename from builtin-http-fetch.c
rename to http-fetch.c
index f3e63d7..e8f44ba 100644
--- a/builtin-http-fetch.c
+++ b/http-fetch.c
@@ -1,8 +1,9 @@
1  #include "cache.h"
2  #include "walker.h"
3
4 -int cmd_http_fetch(int argc, const char **argv, const char *prefix)
4 +int main(int argc, const char **argv)
5  {
6 +       const char *prefix;
7         struct walker *walker;
8         int commits_on_stdin = 0;
@@ -18,6 +19,8 @@ int cmd_http_fetch(int argc, const char **argv, const char *prefix)
19         int get_verbosely = 0;
20         int get_recover = 0;
21
22 +       prefix = setup_git_directory();
23 +
24         git_config(git_default_config, NULL);
25
26         while (arg < argc && argv[arg][0] == '-') {

No unit test is added as I'm not sure how the internal program works.. feel free to decline if it's irrelevant to the project.

[jnareb]

If you provide line numbers, you should provide two columns: for pre-image and for post-image, isn't it?

Or do you decide to provide lines only for post-image - what then to do with deleted / changed lines? Currently it seems like you want pre-image line number... but this might mean non-monotonic line numbers.