[PATCH/RFC] branch: warn user about non-existent branch

[Kaartic Sivaraam <kaarticsivaraam91196@gmail.com>]

The inexistence of the branch trying to be renamed wasn't checked
and was left for 'rename_ref' to point out. It's better to do it
explicitly as it leads to unconventional behaviour in the following
case,

        $ git branch -m foo master
        fatal: A branch named 'master' already exists.

It's conventional to report that the 'foo' doesn't exist rather than
repoting that 'master' exists, the same way the 'mv' command does.

        $ mv foo existing_file
        mv: cannot stat 'foo': No such file or directory

Further, there's no way for 'master' being overwritten with 'foo',
as it doesn't exist. Reporting the existence of 'master' is germane
only when 'master' is *really* going to be overwritten.

So, report the inexistence of the branch explicitly  before reporting
existence of new branch name to be consistent with it's counterpart,
the widely used, the 'mv' command.

Signed-off-by: Kaartic Sivaraam <kaarticsivaraam91196@gmail.com>
---
 I'm sending this patch as I didn't want to leave this thread
 open ended. I'm not yet sure if this is a good thing to do.
 This patch is open to comments, as the prvious ones I've sent
 have been.

 builtin/branch.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/builtin/branch.c b/builtin/branch.c
index a3bd2262b..0a9112335 100644
--- a/builtin/branch.c
+++ b/builtin/branch.c
@@ -473,6 +473,10 @@ static void rename_branch(const char *oldname, const char *newname, int force)
 			die(_("Invalid branch name: '%s'"), oldname);
 	}
 
+	/* Check for existence of oldref before proceeding */
+	if(!ref_exists(oldref.buf))
+		die(_("Branch '%s' does not exist."), oldname);
+
 	/*
 	 * A command like "git branch -M currentbranch currentbranch" cannot
 	 * cause the worktree to become inconsistent with HEAD, so allow it.
-- 
2.13.2.23.g14d9f4c6d