[PATCH] refs.c: interpret @ as HEAD

[Ramkumar Ramachandra <artagnon@gmail.com>]

The rev spec forms @{}, .., ... fill in HEAD as the missing argument
automatically.  Unfortunately, HEAD~<n> is a very common idiom and
there is no way to make HEAD implicit here (due the shell expansion of
~<n>).

However, there is an alternative solution to the issue: overload the
character @ to mean HEAD.  Do this at the lowest possible layer of
abstraction: in dwim_ref(), substitute @ with HEAD just before calling
resolve_ref_unsafe().  The program will only reach this point after
the other specs like ~, ^ and @{} have been resolved; therefore, it is
safe to do it here.

This patch has the exact same effect as:

    $ git symbolic-ref @ HEAD

It means that you can now do @~1, @^2, and even topic..@.  However,
since the @-parsing happens before we ever reach the symref
resolution, @@{u} is invalid.  But this is okay, since @{u} already
has an implicit HEAD in it.

Inspired-by: Felipe Contreras <felipe.contreras@gmail.com>
Inspired-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
---
 I haven't included documentation/ tests because I want feedback on
 this two-liner first.

 refs.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/refs.c b/refs.c
index de2d8eb..cb67b73 100644
--- a/refs.c
+++ b/refs.c
@@ -1604,6 +1604,8 @@ int dwim_ref(const char *str, int len, unsigned char *sha1, char **ref)
 
 		this_result = refs_found ? sha1_from_ref : sha1;
 		mksnpath(fullref, sizeof(fullref), *p, len, str);
+		if (!strcmp(fullref, "@"))
+			mksnpath(fullref, sizeof(fullref), *p, 4, "HEAD");
 		r = resolve_ref_unsafe(fullref, this_result, 1, &flag);
 		if (r) {
 			if (!refs_found++)
-- 
1.8.2.1.628.gcd33b41.dirty