How to find out in which commit a particular code was added?

There is something quicker than issuing a blame on the full file. If the line is ${lineno} and the file is ${filename} you can:

git blame -L ${lineno},${lineno} ${filename}

Example:

git blame -L 2,2 test.txt

git log -S searchTerm

gives you the commits in which the search term was introduced.

git log -S "mention here line of code" [file-path]    

For example:

git log -S "First line" test.txt         

Providing the file name with its path is obvious because, most of the time, we want to know who introduced a particular code segment in a particular file.

Run git blame on the file. It'll show you the commit ID, the date and time, and who committed it- for each line. Then just copy out the commit identifier and you can use it in git log <commit> or git show <commit>.

For example, I've got a file, called test.txt, with lines added on different commits:

$ cat test.txt
First line.
Second line.

Running the git blame:

$ git blame test.txt
^410c3dd (Leigh 2013-11-09 12:00:00 1) First line.
2365eb7d (Leigh 2013-11-09 12:00:10 2) Second line.

The first bit is the commit ID, then name, then date, time, time zone, and finally the line number and line contents.