builtin/checkout: fix `git checkout -p HEAD...` bug

builtin/checkout: fix `git checkout -p HEAD...` bug

Running `git checkout -p` with a merge-base rev results in an error:

	$ git checkout -p HEAD...
	usage: git diff-index [-m] [--cached] [<common-diff-options>] <tree-ish> [<path>...]
	common diff options:
	  -z            output diff-raw with lines terminated with NUL.
	  -p            output patch format.
	  -u            synonym for -p.
	  --patch-with-raw
			output both a patch and the diff-raw format.
	  --stat        show diffstat instead of patch.
	  --numstat     show numeric diffstat instead of patch.
	  --patch-with-stat
			output a patch and prepend its diffstat.
	  --name-only   show only names of changed files.
	  --name-status show names and status of changed files.
	  --full-index  show full object name on index lines.
	  --abbrev=<n>  abbreviate object names in diff-tree header and diff-raw.
	  -R            swap input file pairs.
	  -B            detect complete rewrites.
	  -M            detect renames.
	  -C            detect copies.
	  --find-copies-harder
			try unchanged files as candidate for copy detection.
	  -l<n>         limit rename attempts up to <n> paths.
	  -O<file>      reorder diffs according to the <file>.
	  -S<string>    find filepair whose only one side contains the string.
	  --pickaxe-all
			show all files diff when -S is used and hit is found.
	  -a  --text    treat all files as text.

	Cannot close git diff-index --cached --numstat --summary HEAD... -- () at <redacted>/libexec/git-core/git-add--interactive line 183.

This happens because checkout passes the literal argument (in the
example, `HEAD...`) to diff-index which does not recognise merge-base
revs.

Fix this by using the hex of the found commit instead of the given name.
Note that "HEAD" is handled specially in run_add_interactive() so it's
explicitly not changed.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>