tests/test-merge4.t

lfs: default the User-Agent header for blob transfers to 'git-lfs'

The custom User-Agent for blob transfers was added in e7bb5fc4570c.  Now I've
hit another incompatibility with a server wanting the string to start with
'git' or 'git-lfs' [1].  I don't feel strongly about this either way, but a
Wireshark trace of git shows that when the Batch API is hit, the User-Agent is
'git-lfs/2.3.4'.  So this would probably ensure maximum interoperability.

This still leaves the experimental knob in, just in case.

[1] https://bitbucket.org/sdorra/scm-manager/src/095a027178888bc2b819aebfae3d2c192c858030/scm-plugins/scm-git-plugin/src/main/java/sonia/scm/web/GitUserAgentProvider.java?at=default&fileviewer=file-view-default#GitUserAgentProvider.java-117

  $ hg init
  $ echo This is file a1 > a
  $ hg add a
  $ hg commit -m "commit #0"
  $ echo This is file b1 > b
  $ hg add b
  $ hg commit -m "commit #1"
  $ hg update 0
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
  $ echo This is file c1 > c
  $ hg add c
  $ hg commit -m "commit #2"
  created new head
  $ hg merge 1
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  (branch merge, don't forget to commit)
  $ rm b
  $ echo This is file c22 > c

Test hg behaves when committing with a missing file added by a merge

  $ hg commit -m "commit #3"
  abort: cannot commit merge with missing files
  [255]