How do I ignore files in a directory in Git?

It would be the former. Go by extensions as well instead of folder structure.

I.e. my example C# development ignore file:

#OS junk files
[Tt]humbs.db
*.DS_Store

#Visual Studio files
*.[Oo]bj
*.user
*.aps
*.pch
*.vspscc
*.vssscc
*_i.c
*_p.c
*.ncb
*.suo
*.tlb
*.tlh
*.bak
*.[Cc]ache
*.ilk
*.log
*.lib
*.sbr
*.sdf
ipch/
obj/
[Bb]in
[Dd]ebug*/
[Rr]elease*/
Ankh.NoLoad

#Tooling
_ReSharper*/
*.resharper
[Tt]est[Rr]esult*

#Project files
[Bb]uild/

#Subversion files
.svn

# Office Temp Files
~$*

Update

I thought I'd provide an update from the comments below. Although not directly answering the OP's question, see the following for more examples of .gitignore syntax.

Community wiki (constantly being updated):

.gitignore for Visual Studio Projects and Solutions

More examples with specific language use can be found here (thanks to Chris McKnight's comment):

https://github.com/github/gitignore

Paths which contain slashes are taken to be relative to the directory containing the .gitignore file - usually the top level of your repository, though you can also place them in subdirectories.

So, since in all of the examples you give, the paths contain slashes, the two versions are identical. The only time you need to put a leading slash is when there isn't one in the path already. For example, to ignore foo only at the top level of the repository, use /foo. Simply writing foo would ignore anything called foo anywhere in the repository.

Your wildcards are also redundant. If you want to ignore an entire directory, simply name it:

lib/model/om

The only reason to use wildcards the way you have is if you intend to subsequently un-ignore something in the directory:

lib/model/om/*      # ignore everything in the directory
!lib/model/om/foo   # except foo

PATTERN FORMAT

• A blank line matches no files, so it can serve as a separator for readability.

• A line starting with # serves as a comment.

• An optional prefix ! which negates the pattern; any matching file excluded by a previous pattern will become included again. If a negated pattern matches, this will override lower precedence patterns sources.

• If the pattern ends with a slash, it is removed for the purpose of the following description, but it would only find a match with a directory. In other words, foo/ will match a directory foo and paths underneath it, but will not match a regular file or a symbolic link foo (this is consistent with the way how pathspec works in general in git).

• If the pattern does not contain a slash /, git treats it as a shell glob pattern and checks for a match against the pathname relative to the location of the .gitignore file (relative to the toplevel of the work tree if not from a .gitignore file).

• Otherwise, git treats the pattern as a shell glob suitable for consumption by fnmatch(3) with the FNM_PATHNAME flag: wildcards in the pattern will not match a / in the pathname. For example, Documentation/*.html matches Documentation/git.html but not Documentation/ppc/ppc.html or tools/perf/Documentation/perf.html.

• A leading slash matches the beginning of the pathname. For example, /*.c matches cat-file.c but not mozilla-sha1/sha1.c.

You can find more here

git help gitignore
or
man gitignore