remotefilelog: avoid hard-coding pack names in tests

I'm about to alter the pack format, which alters the content-addressed
name of the pack file. In order to reduce my own confusion I found it
helpful to use an ls | head pipeline since we always want to inspect
the newest pack.

Differential Revision: https://phab.mercurial-scm.org/D4942

tests/test-remotefilelog-bgprefetch.t

@@ -153,7 +153,7 b''
 # Ensure that file 'w' was prefetched - it was not part of the update operation and therefore
 # could only be downloaded by the background prefetch
 
-  $ hg debugdatapack $TESTTMP/hgcache/master/packs/8299d5a1030f073f4adbb3b6bd2ad3bdcc276df0.datapack
+  $ hg debugdatapack `ls -ct $TESTTMP/hgcache/master/packs/*.datapack | head -n 1`
   $TESTTMP/hgcache/master/packs/8299d5a1030f073f4adbb3b6bd2ad3bdcc276df0:
   w:
   Node          Delta Base    Delta Length  Blob Size
@@ -205,7 +205,7 b''
 # Ensure that file 'w' was prefetched - it was not part of the commit operation and therefore
 # could only be downloaded by the background prefetch
 
-  $ hg debugdatapack $TESTTMP/hgcache/master/packs/8299d5a1030f073f4adbb3b6bd2ad3bdcc276df0.datapack
+  $ hg debugdatapack `ls -ct $TESTTMP/hgcache/master/packs/*.datapack | head -n 1`
   $TESTTMP/hgcache/master/packs/8299d5a1030f073f4adbb3b6bd2ad3bdcc276df0:
   w:
   Node          Delta Base    Delta Length  Blob Size
@@ -248,7 +248,7 b''
 # Ensure that file 'y' was prefetched - it was not part of the rebase operation and therefore
 # could only be downloaded by the background prefetch
 
-  $ hg debugdatapack $TESTTMP/hgcache/master/packs/8299d5a1030f073f4adbb3b6bd2ad3bdcc276df0.datapack
+  $ hg debugdatapack `ls -ct $TESTTMP/hgcache/master/packs/*.datapack | head -n 1`
   $TESTTMP/hgcache/master/packs/8299d5a1030f073f4adbb3b6bd2ad3bdcc276df0:
   w:
   Node          Delta Base    Delta Length  Blob Size
@@ -295,7 +295,7 b''
   $TESTTMP/hgcache/repos
 
 # Ensure that files were prefetched
-  $ hg debugdatapack $TESTTMP/hgcache/master/packs/8299d5a1030f073f4adbb3b6bd2ad3bdcc276df0.datapack
+  $ hg debugdatapack `ls -ct $TESTTMP/hgcache/master/packs/*.datapack | head -n 1`
   $TESTTMP/hgcache/master/packs/8299d5a1030f073f4adbb3b6bd2ad3bdcc276df0:
   w:
   Node          Delta Base    Delta Length  Blob Size
@@ -339,7 +339,7 b''
   $TESTTMP/hgcache/repos
 
 # Ensure that files were prefetched
-  $ hg debugdatapack $TESTTMP/hgcache/master/packs/8299d5a1030f073f4adbb3b6bd2ad3bdcc276df0.datapack
+  $ hg debugdatapack `ls -ct $TESTTMP/hgcache/master/packs/*.datapack | head -n 1`
   $TESTTMP/hgcache/master/packs/8299d5a1030f073f4adbb3b6bd2ad3bdcc276df0:
   w:
   Node          Delta Base    Delta Length  Blob Size

tests/test-remotefilelog-gcrepack.t

@@ -50,7 +50,7 b''
 
 # Ensure that all file versions were prefetched
 
-  $ hg debugdatapack $TESTTMP/hgcache/master/packs/f7a942a6e4673d2c7b697fdd926ca2d153831ca4.datapack
+  $ hg debugdatapack `ls -ct $TESTTMP/hgcache/master/packs/*.datapack | head -n 1`
   $TESTTMP/hgcache/master/packs/f7a942a6e4673d2c7b697fdd926ca2d153831ca4:
   x:
   Node          Delta Base    Delta Length  Blob Size
@@ -84,7 +84,7 b''
 # Ensure that file 'x' was garbage collected. It should be GCed because it is not in the keepset
 # and is old (commit date is 0.0 in tests). Ensure that file 'y' is present as it is in the keepset.
 
-  $ hg debugdatapack $TESTTMP/hgcache/master/packs/05baa499c6b07f2bf0ea3d2c8151da1cb86f5e33.datapack
+  $ hg debugdatapack `ls -ct $TESTTMP/hgcache/master/packs/*.datapack | head -n 1`
   $TESTTMP/hgcache/master/packs/05baa499c6b07f2bf0ea3d2c8151da1cb86f5e33:
   y:
   Node          Delta Base    Delta Length  Blob Size
@@ -111,7 +111,7 b''
 
 # Ensure that all file versions were prefetched
 
-  $ hg debugdatapack $TESTTMP/hgcache/master/packs/f7a942a6e4673d2c7b697fdd926ca2d153831ca4.datapack
+  $ hg debugdatapack `ls -ct $TESTTMP/hgcache/master/packs/*.datapack | head -n 1`
   $TESTTMP/hgcache/master/packs/f7a942a6e4673d2c7b697fdd926ca2d153831ca4:
   x:
   Node          Delta Base    Delta Length  Blob Size
@@ -145,7 +145,7 b''
 
 # Ensure that all file versions were prefetched
 
-  $ hg debugdatapack $TESTTMP/hgcache/master/packs/f7a942a6e4673d2c7b697fdd926ca2d153831ca4.datapack
+  $ hg debugdatapack `ls -ct $TESTTMP/hgcache/master/packs/*.datapack | head -n 1`
   $TESTTMP/hgcache/master/packs/f7a942a6e4673d2c7b697fdd926ca2d153831ca4:
   x:
   Node          Delta Base    Delta Length  Blob Size