Add support for GIT_CEILING_DIRECTORIES

Make git recognize a new environment variable that prevents it from
chdir'ing up into specified directories when looking for a GIT_DIR.
Useful for avoiding slow network directories.

For example, I use git in an environment where homedirs are automounted
and "ls /home/nonexistent" takes about 9 seconds.  Setting
GIT_CEILING_DIRS="/home" allows "git help -a" (for bash completion) and
"git symbolic-ref" (for my shell prompt) to run in a reasonable time.

Signed-off-by: David Reiss <dreiss@facebook.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t0060-path-utils.sh b/t/t0060-path-utils.sh
index 9076b3b..6e7501f 100755
--- a/t/t0060-path-utils.sh
+++ b/t/t0060-path-utils.sh
@@ -12,6 +12,11 @@
 	"test \$(test-path-utils normalize_absolute_path '$1') = '$2'"
 }
 
+ancestor() {
+	test_expect_success "longest ancestor" \
+	"test \$(test-path-utils longest_ancestor_length '$1' '$2') = '$3'"
+}
+
 norm_abs "" /
 norm_abs / /
 norm_abs // /
@@ -37,4 +42,46 @@
 norm_abs /d1/.../d2 /d1/.../d2
 norm_abs /d1/..././../d2 /d1/d2
 
+ancestor / "" -1
+ancestor / / -1
+ancestor /foo "" -1
+ancestor /foo : -1
+ancestor /foo ::. -1
+ancestor /foo ::..:: -1
+ancestor /foo / 0
+ancestor /foo /fo -1
+ancestor /foo /foo -1
+ancestor /foo /foo/ -1
+ancestor /foo /bar -1
+ancestor /foo /bar/ -1
+ancestor /foo /foo/bar -1
+ancestor /foo /foo:/bar/ -1
+ancestor /foo /foo/:/bar/ -1
+ancestor /foo /foo::/bar/ -1
+ancestor /foo /:/foo:/bar/ 0
+ancestor /foo /foo:/:/bar/ 0
+ancestor /foo /:/bar/:/foo 0
+ancestor /foo/bar "" -1
+ancestor /foo/bar / 0
+ancestor /foo/bar /fo -1
+ancestor /foo/bar foo -1
+ancestor /foo/bar /foo 4
+ancestor /foo/bar /foo/ 4
+ancestor /foo/bar /foo/ba -1
+ancestor /foo/bar /:/fo 0
+ancestor /foo/bar /foo:/foo/ba 4
+ancestor /foo/bar /bar -1
+ancestor /foo/bar /bar/ -1
+ancestor /foo/bar /fo: -1
+ancestor /foo/bar :/fo -1
+ancestor /foo/bar /foo:/bar/ 4
+ancestor /foo/bar /:/foo:/bar/ 4
+ancestor /foo/bar /foo:/:/bar/ 4
+ancestor /foo/bar /:/bar/:/fo 0
+ancestor /foo/bar /:/bar/ 0
+ancestor /foo/bar :://foo/. 4
+ancestor /foo/bar :://foo/.:: 4
+ancestor /foo/bar //foo/./::/bar 4
+ancestor /foo/bar ::/bar -1
+
 test_done