[PATCH 3/3] reject @{-1} not at beginning of object name

Something like foo@{-1} is nonsensical, as the @{-N} syntax
is reserved for "the Nth last branch", and is not an actual
reflog selector. We should not feed such nonsense to
approxidate at all.

Signed-off-by: Jeff King <redacted>
---
We didn't discuss this one, but I came across it while trying to be
complete in testing the combinations. Right now "foo@{-1}" is
interpreted as a reflog entry at approxidate "-1". Approxidate doesn't
signal an error because it thinks it has found something useful. But
AFAIK we have declared all @{-...} to be Nth last branch, so it is
simply a semantic error.

Let me know if that is not the case (that is, if it was intentional to
leave foo@{-1} as the reflog at date "-1" because it has some meaning
that I am missing) and we can drop this patch.

 sha1_name.c                |    4 ++++
 t/t1508-at-combinations.sh |    2 +-
 2 files changed, 5 insertions(+), 1 deletions(-)

diff --git a/sha1_name.c b/sha1_name.c
index 00fc415..7729925 100644
--- a/sha1_name.c
+++ b/sha1_name.c

@@ -399,6 +399,10 @@ static int get_sha1_basic(const char *str, int len, unsigned char *sha1)
 		unsigned long co_time;
 		int co_tz, co_cnt;
 
+		/* a @{-N} placed anywhere except the start is an error */
+		if (str[at+2] == '-')
+			return -1;
+
 		/* Is it asking for N-th entry, or approxidate? */
 		for (i = nth = 0; 0 <= nth && i < reflog_len; i++) {
 			char ch = str[at+2+i];

diff --git a/t/t1508-at-combinations.sh b/t/t1508-at-combinations.sh
index 2a46af2..d5d6244 100755
--- a/t/t1508-at-combinations.sh
+++ b/t/t1508-at-combinations.sh

@@ -45,7 +45,7 @@ check "@{u}" upstream-two
 check "@{u}@{1}" upstream-one
 check "@{-1}@{u}" master-two
 check "@{-1}@{u}@{1}" master-one
-fail nonsense "@{u}@{-1}"
+nonsense "@{u}@{-1}"
 nonsense "@{1}@{u}"
 
 test_done

-- 
1.7.0.rc0.41.g538720